gallium/st: add non-CSC lowering of I420 as PIPE_FORMAT_R8_G8_B8_420

gallium/st: add non-CSC lowering of I420 as PIPE_FORMAT_R8_G8_B8_420

This new format is similar to PIPE_FORMAT_G8_B8_R8_420, but with R as Y, G as U
and B as V. The need for two diferent formats here is because gallium maps the
YUV channels differently from vulkan.

Some hardware, e.g. Mali GPUs, can sample from I420 but need CSC in shader,
this patch implements that.

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>