drm: Remove drm_fb_helper add, add all and remove connector calls

drm_fb_helper_{add,remove}_one_connector() and
drm_fb_helper_single_add_all_connectors() are dummy functions now
and serve no purpose. Hence remove their calls.

This is the preparatory step for removing the
drm_fb_helper_{add,remove}_one_connector() functions from
drm_fb_helper.h

This removal is done using below sementic patch and unused variable
compilation warnings are fixed manually.

@@
@@

- drm_fb_helper_single_add_all_connectors(...);

@@
expression e1;
statement S;
@@
- e1 = drm_fb_helper_single_add_all_connectors(...);
- S

@@
@@

- drm_fb_helper_add_one_connector(...);

@@
@@

- drm_fb_helper_remove_one_connector(...);

Changes since v1:
* Squashed warning fixes into the patch that introduced the
  warnings (into 5/7) (Laurent, Emil, Lyude)

Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305120434.111091-6-pankaj.laxminarayan.bharadiya@intel.com