drm/imx: Use drm_fb_cma_fbdev_init/fini()

Use drm_fb_cma_fbdev_init() and drm_fb_cma_fbdev_fini() which relies on
the fact that drm_device holds a pointer to the drm_fb_helper structure.
This means that the driver doesn't have to keep track of that.
Also use the drm_fb_helper functions directly.

Cc: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115142001.45358-8-noralf@tronnes.org