winsys/amdgpu: Retrieve WC flags from imported buffers.

winsys/amdgpu: Retrieve WC flags from imported buffers.

Otherwise reading from an imported mapped GTT+WC linear texture
is painfully slow.

Sadly no radeon winsys implementation, as I don't know a suitable
kernel driver operation.

Hit this  in vaGetImage with an image imported from minigbm (which
we are switching to allocate WC for SCANOUT images).

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4542>