gallium: Add a cap for enabling lowering of image load/store intrinsics.

gallium: Add a cap for enabling lowering of image load/store intrinsics.

The deref stuff is hard to handle in a backend supporting dynamic
indexing, while the lowering can easily turn that into the same kind of
dynamic indexing we do for textures, UBOs, and SSBOs.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3728>