gallium/st: add non-CSC lowering of YV12 as PIPE_FORMAT_R8_B8_G8_420

gallium/st: add non-CSC lowering of YV12 as PIPE_FORMAT_R8_B8_G8_420

YV12 is the same as DRM_FORMAT_YVU420.
We lower it to PIPE_FORMAT_R8_B8_G8_420, which is equivalent to
PIPE_FORMAT_R8_G8_B8_420 with U/V planes swapped.

This is used for hardware that can sample from YUV but need CSC in shader.

Signed-off-by: Italo Nicola <italonicola@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24266>