drm/amd/display: fix HDMI 12bits last two bits are zero issue

(FF-158) -[FF N-1] HDMI Deep Color Mode incorrect bit depth:
FMT_DYNAMIC_EXP_EN is being reset to 0

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>