gma500: remove unneeded check in mdfld_crtc_mode_set()

The list cursor is never NULL in a list_for_each_entry() loop.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>