drm/amd/display: Properly handle additional cases where DCN is not supported

There could be boards with DCN listed in IP discovery, but no
display hardware actually wired up.  In this case the vbios
display table will not be populated.  Detect this case and
skip loading DM when we detect it.

v2: Mark DCN as harvested as well so other display checks
elsewhere in the driver are handled properly.

Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>