st/mesa: don't lower YUV when driver supports it natively

This fixes YUYV support on etnaviv.

Fixes: 7404833c "gallium: add handling for YUV planar surfaces"

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/1896>
6 files changed