drm/gm12u320: More drmm_

The drm_mode_config_cleanup call we can drop, and all the allocations
we can switch over to drmm_kzalloc. Unfortunately the work queue is
still present, so can't get rid of the drm_driver->release function
outright.

v2: Use drmm_mode_config_init() for more clarity (Sam, Thomas)

Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-43-daniel.vetter@ffwll.ch