i965/fs: Rework representation of fragment output locations in NIR.

The problem with the current approach is that driver output locations
are represented as a linear offset within the nir_outputs array, which
makes it rather difficult for the back-end to figure out what color
output and index some nir_intrinsic_load/store_output was meant for,
because the offset of a given output within the nir_output array is
dependent on the type and size of all previously allocated outputs.
Instead this defines the driver location of an output to be the pair
formed by its GLSL-assigned location and index (I've borrowed the
bitfield macros from brw_defines.h in order to represent the pair of
integers as a single scalar value that can be assigned to
nir_variable_data::driver_location).  nir_assign_var_locations is no
longer useful for fragment outputs.

Because fragment outputs are now allocated independently rather than
within the nir_outputs array, the get_frag_output() helper becomes
necessary in order to obtain the right temporary register for a given
location-index pair.

The type_size helper passed to nir_lower_io is now type_size_dvec4
rather than type_size_vec4_times_4 so that output array offsets are
provided in terms of whole array elements rather than in terms of
scalar components (dvec4 is the largest vector type supported by the
GLSL so this will cause all individual fragment outputs to have a size
of one regardless of the type).

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

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