drm/amd/display: Add enum for H-timing divider mode

Add h_timing_div_mode enum to better reflect possible register
values. Replace previously programmed values with enum

Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>