drm/amd/display: add debug option for dramclk_change_latency in apu

[Why & How]
Support dramclk change latency change via debug option and add some
code isolation.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Charlene Liu <Charlene.Liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>