v3d/tex: avoid to ask back for a sampler state if not needed

v3d/tex: avoid to ask back for a sampler state if not needed

So far we were not asking the driver for the sampler state if we could
just use the default P1 values. But even if we need to fill P1 (for
example to fill up the output type of the format), if the texture
operation doesn't need a sampler, we can let that field as NULL (so
default values) and avoid calling back the driver for a sampler.

This is not mandatory for OpenGL (as we always have a sampler object),
although still a good to have. For Vulkan this is needed, as we don't
have a sampler object in that case.

v2: reword comment (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>