i965/fs: Stop wasting input attribute space on gl_FragCoord and gl_FrontFacing.

i965/fs: Stop wasting input attribute space on gl_FragCoord and gl_FrontFacing.

Previously, if a fragment shader accessed gl_FragCoord or
gl_FrontFacing, we would assign them their own slots in the fragment
shader input attribute array, using up space that could be made
available to real varyings.  This was not strictly necessary (since
these values are not true varyings, and are instead computed from
other data available in the FS payload).  But we had to do it anyway
because the SF/SBE setup code assumed that every 1 bit in the
gl_program::InputsRead bitfield corresponded to a genuine varying
variable.

Now that the SF/SBE code consults brw_wm_prog_data and only sets up
the attributes that the fragment shader actually needs, we don't have
to do this anymore.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>