drm/amd/display: Preserve gpu memory allocation for life of dc

[Why]
We want to keep the same buffer allocated for use during multiple
hardware initializations.

[How]
 - allocate gpu memory buffer on clock manager construct
 - free gpu memory buffer on clock manager destruct

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Eryk Brol <eryk.brol@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>