drm/amd/display: rename core_dc to dc

[Why]
First, to make code more consistent
Second, to get rid of those scenario where we create a second
local pointer to dc when it's already passed in.

[How]
Rename core_dc to dc
Remove duplicate local pointers to dc

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>