v3dv: remove combined_idx support

v3dv: remove combined_idx support

Now that the v3d compiler has support for separated texture and
sampler indices, we can stop to combine them. Again, that's what
Vulkan allows after all.

As we are doing this we can't use anymore the texture format (coming
from the texture) to chose the return size (that is a sampling
parameter). We default for 32, and just go to 16 for shadow. We plan
to use SPIR-V RelaxedPrecision to use in more cases 16 bit. We would
do that on following patches.

v2 (from Iago feedback):
   * Fix typos/bad grammar on comments.
   * Move tex/sampler number assert to before the loop that fills
     tex/sampler info.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7545>