Revert "drm/panfrost: Use drm_gem_map_offset()"

This reverts commit 583bbf46.

Turns out we need mmap to work on imported BOs even if the current code
is buggy.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807145253.2037-3-sean@poorly.run