drm/vgem: Drop vgem_drm_gem_mmap

drm/vgem: Drop vgem_drm_gem_mmap

It's duplicating (without using some of the helpers) drm_gem_mmap with
the addition that it can redirect to drm-buf mmap support. But prime
import/export was dropped in

commit 990ed2720717173bbdea4cfb2bad37cc7aa91495
Author: Rob Clark <robdclark@gmail.com>
Date:   Thu May 21 11:58:30 2015 -0400

    drm/vgem: drop DRIVER_PRIME (v2)

for now, so this is dead code. And since I want to rework the locking
for drm_gem_mmap it seems simpler to de-dupe this code for now and
then start over with the reworked one again, if we want to resurrect
this all indeed.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://mid.gmane.org/1444894601-5200-8-git-send-email-daniel.vetter@ffwll.ch
Acked-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>