drm/cma: Fix compile fail due to fomat->format typo

drm/cma: Fix compile fail due to fomat->format typo

Apparently my arm .config had reverted to CMA=n at some point, so I
failed to notice that I typoed the code. Fix it up so that the
cma helper will compile again.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Fixes: ca984a998ad3 ("drm/fb_cma_helper: Replace drm_format_info() with fb->format")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161215142927.20761-1-ville.syrjala@linux.intel.com
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>