drm/amd/display: Handle DCE 6 in dce_clk_mgr.c

drm/amd/display: Handle DCE 6 in dce_clk_mgr.c

dce60_clk_mgr was basically identical to dce_clk_mgr other than
a few minor details. They can be all handled in the same file,
reducing duplicated code and easing the maintenance burden for
old DCE versions.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rodrigo Siqueira <siqueira@igalia.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>