drm/amd/display: Drop unnecessary header

A long time ago, the slab header was added to multiple files in DC. We
also included it in the os_types.h, which is included in many of those
DC files. At this point, there is no need to insert the slab.h header in
multiple files, so this commit drops those includes.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>