freedreno/a6xx: Fix NV12+UBWC import
What does this MR do and why?
freedreno/a6xx: Fix NV12+UBWC import
Treat R8_G8B8_420_UNORM and NV12 the same, because dri2 frontend doesn't
understand or care about the difference from the sampler PoV.
Fixes: 1e820ac12850 ("freedreno: Rework supported-modifiers handling")
Signed-off-by: Rob Clark <robdclark@chromium.org>