drm/panel: add drm_connector argument to get_modes()

Today the bridge creates the drm_connector, but that is planned
to be moved to the display drivers.
To facilitate this, update drm_panel_funcs.get_modes() to
take drm_connector as an argument.
All panel drivers implementing get_modes() are updated.

v2:
  - drop accidental change (Laurent)
  - update docs for get_modes (Laurent)

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Stefan Mavrodiev <stefan@olimex.com>
Cc: Robert Chiras <robert.chiras@nxp.com>
Cc: "Guido Günther" <agx@sigxcpu.org>
Cc: Purism Kernel Team <kernel@puri.sm>
Link: https://patchwork.freedesktop.org/patch/msgid/20191207140353.23967-6-sam@ravnborg.org