drm/amd/display: refactor reg_update

[why]
separate register access from logic to allow us abstract register sequences

[how]
consolidate mask and value first then apply to register.

Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>