drm/rockchip: use drm_gem_mmap helpers

drm/rockchip: use drm_gem_mmap helpers

commit 41315b793e13f884cda79389f0d5d44d027e57d1 upstream.

Rather than (incompletely [0]) re-implementing drm_gem_mmap() and
drm_gem_mmap_obj() helpers, call them directly from the rockchip mmap
routines.

Once the core functions return successfully, the rockchip mmap routines
can still use dma_mmap_attrs() to simply mmap the entire buffer.

[0] Previously, we were performing the mmap() without first taking a
reference on the underlying gem buffer.  This could leak ptes if the gem
object is destroyed while userspace is still holding the mapping.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>