drm/amd/display: Fix gamma colour corruption for 10 bit surfaces

Gamma LUT shouldn't be used for 10-bit and above. Should instead be
using prescale and bypassing input gamma.

Signed-off-by: Jordan Lazare <Jordan.Lazare@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>