drm: Drop drm_helper_probe_single_connector_modes_nomerge()

Now that the mode type bit merge logic is fixed to only merge
between new probed modes, hopefully we can eliminat the special
case for qxl and virtio. That is make the merge the mode type
bits from all matching new probed modes, just like every other
driver.

qxl and virtio got excluded from the merging in
commit 3fbd6439 ("drm: copy mode type in drm_mode_connector_list_update()")
commit abce1ec9 ("Revert "drm: copy mode type in drm_mode_connector_list_update()"")
commit b87577b7 ("drm: try harder to avoid regression when merging mode bits")

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Adam Jackson <ajax@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Resolve conflicts with doc updates.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>