drm/amd/display: Refactor reg_set and reg_update.

[Why]
Current reg update and reg set use same functions and
only delta is update reads reg value and call update function.

[How]
Refactor reg update and reg set functions.
1.Implement different functions for reg update and reg set.
2.Wrap same process to a help function, both reg update and
reg set will call it.

Signed-off-by: Yongqiang Sun <yongqiang.sun@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>