i965: skip reading unused slots at the begining of the URB for the FS

We can start reading the URB at the first offset that contains varyings
that are actually read in the URB. We still need to make sure that we
read at least one varying to honor hardware requirements.

This helps alleviate a problem introduced with 99df02ca26f61 for
separate shader objects: without separate shader objects we assign
locations sequentially, however, since that commit we have changed the
method for SSO so that the VUE slot assigned depends on the number of
builtin slots plus the location assigned to the varying. This fixed
layout is intended to help SSO programs by avoiding on-the-fly recompiles
when swapping out shaders, however, it also means that if a varying uses
a large location number close to the maximum allowed by the SF/FS units
(31), then the offset introduced by the number of builtin slots can push
the location outside the range and trigger an assertion.

This problem is affecting at least the following CTS tests for
enhanced layouts:

KHR-GL45.enhanced_layouts.varying_array_components
KHR-GL45.enhanced_layouts.varying_array_locations
KHR-GL45.enhanced_layouts.varying_components
KHR-GL45.enhanced_layouts.varying_locations

which use SSO and the the location layout qualifier to select such
location numbers explicitly.

This change helps these tests because for SSO we always have to include
things such as VARYING_SLOT_CLIP_DIST{0,1} even if the fragment shader is
very unlikely to read them, so by doing this we free builtin slots from
the fixed VUE layout and we avoid the tests to crash in this scenario.

Of course, this is not a proper fix, we'd still run into problems if someone
tries to use an explicit max location and read gl_ViewportIndex, gl_LayerID or
gl_CullDistancein in the FS, but that would be a much less common bug and we
can probably wait to see if anyone actually runs into that situation in a real
world scenario before making the decision that more aggresive changes are
required to support this without reverting 99df02ca26f61.

v2:
- Add a debug message when we skip clip distances (Ilia)
- we also need to account for this when we compute the urb setup
  for the fragment shader stage, so add a compiler util to compute
  the first slot that we need to read from the URB instead of
  replicating the logic in both places.

v3:
- Make the util more generic so it can account for all unused slots
  at the beginning of the URB, that will make it more useful (Ken).
- Drop the debug message, it was not what Ilia was asking for.

Suggested-by: Kenneth Graunke <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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