drm/imx: Use drm_gem_fb_create() and drm_gem_fb_prepare_fb()

drm_fb_cma_create() and drm_fb_cma_prepare_fb() are just wrappers now,
use drm_gem_fb_create() and drm_gem_fb_prepare_fb() 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/1502631125-13557-11-git-send-email-noralf@tronnes.org