drm/msm: Delete an unnecessary check before drm_gem_object_unreference()

The drm_gem_object_unreference() 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: Rob Clark <robdclark@gmail.com>