drm: Reject fbs w/o format info in drm_framebuffer_init()

Any framebuffer that doesn't have proper format information when
drm_framebuffer_init() is called is a bug. Let's warn and return
an error to avoid oopsing the kernel later due to dereferencing the
NULL fb->format pointer.

Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-23-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>