drm/amd/display: Convert tiling_flags to modifiers.

This way the modifier path gets exercised all the time, improving
testing. Furthermore, for modifiers this is required as getfb2
will always return the modifier if the driver sets allow_fb_modifiers.

This only triggers once allow_fb_modifiers is set.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>