drm/arcpgu: Delete an unnecessary check before drm_fbdev_cma_hotplug_event()

The drm_fbdev_cma_hotplug_event() 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: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/dd9a16b1-234c-6776-e6d9-943a8457334e@users.sourceforge.net