drm/omap: fix suspend/resume handling

For legacy reasons omapdss handles system suspend/resume via PM notifier
callback, where the driver disables/resumes all the outputs.

This doesn't work well with omapdrm. What happens on suspend is that the
omapdss disables the displays while omapdrm is still happily continuing
its work, possibly waiting for an vsync irq, which will never come if
the display output is disabled, leading to timeouts and errors sent to
userspace.

This patch moves the suspend/resume handling to omapdrm, and the
suspend/resume is now done safely inside modeset lock.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>