drm/vgem: Fix mmaping

The vGEM mmap code has bitrotted slightly and now immediately BUGs.
Since vGEM was last updated, there are new core GEM facilities to
provide more common functions, so let's use those here.

v2: drm_gem_free_mmap_offset() is performed from
drm_gem_object_release() so we can remove the redundant call.

Testcase: igt/vgem_basic/mmap
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96603


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Zach Reizner <zachr@google.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Tested-by: Humberto Israel Perez Rodriguez <humberto.i.perez.rodriguez@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466692534-28303-1-git-send-email-chris@chris-wilson.co.uk