i965: Simplify gl_SampleID setup on Gen8+.

On Gen7+, the thread payload provides the sample ID - we can read it
in two instructions, without any elaborate calculations.  We don't even
need a state dependency - this will properly produce zero in the
non-MSAA case.  Unfortunately, we need the state flag anyway, so we
may as well continue to use it to produce a single MOV 0 instead of
SHR/AND.

For some reason, the sample ID field is always zero on Gen7/7.5, so
we can't use this yet.  However, it works fine on Gen8+.  So, land the
code and use it where it's working, and leave a TODO for later.

v2: Fix register types in the comment (caught by Matt Turner!).

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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