This is needed due Vulkan because by spec (31.1. Limit Requirements)
the minimum value for the following limits are the following ones:
  maxPerStageDescriptorSampledImages 16
  maxPerStageDescriptorStorageImages  4
  maxPerStageDescriptorInputAttachments 4

And we are using v3d textures for all of them, so current limit would
not be enough for some cases.

Note that as the current comment explains there is not exactly a HW
limit for it, so we could bump to 32 for example, but let's just be
conservative and ask the minimum required.

It is worth to note that we needed to maintain the same value for the
OpenGL case, as it gets a register allocation failure on some GL
cases. We tried to fix that with small changes on the nir scheduler,
but we found that it would require some non-trivial effort to get it
done (that eventually we would need to).

Fixes tests like:
dEQP-VK.binding_model.descriptorset_random.sets16.constant.ubolimitlow.sbolimitlow.imglimitlow.noiub.uab.comp.noia.0

v2: keep the previous limit for Opengl (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <mesa/mesa!6999>