drm: Add Grain Media GM12U320 driver v2

Add a modesetting driver for Grain Media GM12U320 based devices
(primarily Acer C120 projector, but there may be compatible devices).

This is based on the fb driver from Viacheslav Nurmekhamitov:
https://github.com/slavrn/gm12u320

This driver uses drm_simple_display_pipe to deal with all the atomic
stuff, gem_shmem_helper functions for buffer management and
drm_fbdev_generic_setup for fbdev emulation, so that leaves the driver
itself with only the actual code for talking to the gm12u320 chip,
leading to a nice simple and clean driver.

Changes in v2:
-Add drm-misc tree to MAINTAINERS
-Drop mode_config.preferred_depth = 24 / fix fbdev support

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190721132525.10396-1-hdegoede@redhat.com