drm/gem: dont init "ret" in drm_gem_mmap()

There is no need to initialize this variable, so drop it. Otherwise, the
compiler won't warn if we use it unintialized.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

diff --cc drivers/gpu/drm/drm_gem.c
Simple merge