drm/amd/display: Support FP16 pixel format

[Why]
FP16 pixel format is not declared to DRM in Linux DM.

[How]
Add FP16 format to the support list presented to DRM from Linux DM.

Signed-off-by: Stylon Wang <stylon.wang@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>