intel/fs: avoid reusing the VGRF for uniform load_ubo

intel/fs: avoid reusing the VGRF for uniform load_ubo

Only found 3 shaders affected in Red Dead Redemption :

Totals from 3 (0.05% of 5969) affected shaders:
Instrs: 2246 -> 2230 (-0.71%)
Cycles: 156506 -> 148402 (-5.18%); split: -5.23%, +0.05%

This will have a larger effect when we add the
load_ubo_uniform_block_intel intrinsic where we will have larger
blocks (vec8/vec16 vs vec4 only now).

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23477>