kgsl/drm: add drm_dev in private pointer.

This patch adds drm_dev information into private drv pointer.

Change-Id: Ie51f383af3d98e2ed118ce6fb85cdb4b6cba0646
Signed-off-by: Eunchul Kim <chulspro.kim@samsung.com>
Signed-off-by: Mathew Karimpanal <kjmathew@codeaurora.org>
(cherry picked from commit a52a9f59435fdc947b0bc952a30687449e7316ec)