Restore old dpms state in pm_resume(). The dpms is turned off in
pm_suspend() and it should be restored to its original state in
pm_resume(). Without this patch the display is left blanked after a
suspend/resume cycle.

Fixes commit 614b3cfe ("drm/tilcdc: disable the lcd controller/dma
engine when suspend invoked")
Signed-off-by: Jyri Sarha <jsarha@ti.com>