GPU-DRM-IMX: Delete an unnecessary check before drm_fbdev_cma_restore_mode()

The drm_fbdev_cma_restore_mode() function tests whether its argument
is NULL and then returns immediately.
Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>