drm/amd/display: Expose support for xBGR ordered fp16 formats.

Expose support for DRM_FORMAT_ABGR16161616F and
DRM_FORMAT_XBGR16161616F to the DRM core, complementing
the already existing xRGB ordered fp16 formats.

These are especially useful for creating presentable
swapchains in Vulkan for VK_FORMAT_R16G16B16A16_SFLOAT.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>