drm/gem: Add drm_gem_dumb_map_offset()

Add a common drm_driver.dumb_map_offset function for GEM backed drivers.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500837417-40580-2-git-send-email-noralf@tronnes.org