vulkan/wsi: Add a driconf option to force WSI to advertise BGRA8_UNORM first

vulkan/wsi: Add a driconf option to force WSI to advertise BGRA8_UNORM first

The Aztec Ruins benchmark just grabs the first format in the list and
SRGB causes it to render washed out.  With this workaround, it renders
the same as OpenGL.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3350>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3350>