Revert "Revert "drm/panfrost: Use drm_gem_map_offset()""

This reverts commit be855382.

Mandatory review was missing from this patch.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190807142101.251400-3-sean@poorly.run