drm/vmwgfx: Drop drm_vblank_cleanup

Again stopping the vblank before uninstalling the irq handler is kinda
the wrong way round, but the fb_off stuff should take care of
disabling the dsiplay at least in most cases. So drop the
drm_vblank_cleanup code since it's not really doing anything, it looks
all cargo-culted.

v2: Appease gcc better.

v3: Simplify code (Sean Paul)

Cc: Sinclair Yeh <syeh@vmware.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170626161949.25629-1-daniel.vetter@ffwll.ch