drm/tilcdc: disable the lcd controller/dma engine when suspend invoked

The LCD controller must be deactivated and all DMA transactions stopped
when the suspend power state is entered otherwise the PRCM causes the L3
bus to get stuck in transition state.

This commit forces the lcdc to be shut down and waits for all pending DMA
transactions to complete as part of the suspend handler for this driver.

Signed-off-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>