drm/amd/display: remove get_position() of underlay

get_position() returns CRTC vertical/horizontal counters,
hence it is not applicable for underlay which is plane and not
attached to one crtc.
This patch hence removes the callback from dce110_tg_v_funcs
and hence fixes a bug in dc_debug.c where in
get_position() for underlay pipe is called un-conditionally.

Witout this patch get_position() is called in case one has
to update underlay plane's data,
since callback implmented with an ASSERT(),
the old_surface_count and new_surface_count mismatches; causing
only one plane update which is not right and also assert makes the
rendering slow.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>