drm/gma500: Remove null check before kfree

kfree on NULL pointer is a no-op and therefore checking is redundant.

Signed-off-by: Himanshu Jha <himanshujha199640@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1504099556-3887-1-git-send-email-himanshujha199640@gmail.com