drm/imx: Use drm_fbdev_generic_setup()

The CMA helper is already using the drm_fb_helper_generic_probe part of
the generic fbdev emulation. This patch makes full use of the generic
fbdev emulation by using its drm_client callbacks. This means that
drm_mode_config_funcs->output_poll_changed and drm_driver->lastclose are
now handled by the emulation code. Additionally fbdev unregister happens
automatically on drm_dev_unregister().

The drm_fbdev_generic_setup() call is put after drm_dev_register() in the
driver. This is done to highlight the fact that fbdev emulation is an
internal client that makes use of the driver, it is not part of the
driver as such. If fbdev setup fails, an error is printed, but the driver
succeeds probing.

CONFIG_DRM_FBDEV_EMULATION wasn't honoured by the CMA helper, but it is by
drm_fb_helper.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180908134648.2582-12-noralf@tronnes.org