drm/amd/display: Expose support for DRM_FORMAT_RGB565

DC and DM already support DRM_FORMAT_RGB565, it's just missing from the
list of valid formats.

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -4188,6 +4188,7 @@ static const uint32_t rgb_formats[] = {
 	DRM_FORMAT_ABGR2101010,
 	DRM_FORMAT_XBGR8888,
 	DRM_FORMAT_ABGR8888,
+	DRM_FORMAT_RGB565,
 };
 
 static const uint32_t overlay_formats[] = {