drm/amd/display: Improve warning log for get OPP for OTG master

If some part of the driver tries to call
resource_get_opp_heads_for_otg_master in a non-OTG master context, DC
will trigger a dmesg warning since this situation indicates that some
configuration associated with ODM slices might be wrong. This commit
adds an extra log to describe why the warning was triggered to make the
debugging more straightforward.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Acked-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>