nir: add support for gather offsets

nir: add support for gather offsets

Values inside the offsets parameter of textureGatherOffsets are required to be
constants in the range of [GL_MIN_PROGRAM_TEXTURE_GATHER_OFFSET,
GL_MAX_PROGRAM_TEXTURE_GATHER_OFFSET].

As this range is never outside [-32, 31] for all existing drivers inside mesa,
we can simply store the offsets as a int8_t[4][2] array inside nir_tex_instr.

Right now only Nvidia hardware supports this in hardware, so we can turn this
on inside Nouveau for the NIR path as it is already enabled with the TGSI one.

v2: use memcpy instead of for loops
    add missing bits to nir_instr_set
    don't show offsets if they are all 0
v3: default offsets aren't all 0
v4: rename offsets -> tg4_offsets
    rename nir_tex_instr_has_explicit_offsets -> nir_tex_instr_has_explicit_tg4_offsets

Signed-off-by: Karol Herbst <kherbst@redhat.com>