imx-drm: imx-drm-core: add basic suspend/resume support

HDMI currently stops working after a system suspend/resume cycle.  The
cause is that the mode setting states in hardware gets lost and isn't
restored across the suspend/resume cycle.

The patch adds a very basic suspend/resume support to imx-drm driver,
and calls drm_helper_resume_force_mode() in .resume hook to restore the
mode setting states, so that HDMI can continue working after a system
suspend/resume cycle.

Since the suspend/resume hook can be called with drm_device pointer
being NULL from driver data, we need a check on the pointer in the
hooks.  And to avoid using a stale pointer from driver data, it also
clears driver data in .unload hook.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>