drm/panfrost: Use drm_gem_map_offset()

panfrost_ioctl_mmap_bo() contains a reimplementation of
drm_gem_map_offset() but with a bug - it allows mapping imported
objects (without going through the exporter). Fix this by switching to
use the newly renamed drm_gem_map_offset() function instead which has
the bonus of simplifying the code.

Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190627155318.38053-3-steven.price@arm.com