drm/amd/display: Refactor clk_mgr functions

[Why]
Some HW specific implementations can be pulled out into clk_mgr.c.

[How]
- Pull get_active_display_cnt out to clk_mgr.
- Pull out shared logic in set_dispclk and set_dprefclk

Signed-off-by: Eric Yang <Eric.Yang2@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>