drm/omap: stop connector polling during suspend

When not using proper hotplug detection, DRM polls periodically the
connectors to find out if a cable is connected. This polling can happen
at any time, even very late in the suspend process.

This causes a problem with omapdrm, when the poll happens during the
suspend process after GPIOs have been disabled, leading to a crash in
gpio_get().

This patch fixes the issue by adding suspend and resume hooks to
omapdrm, in which we disable and enable, respectively, the polling.

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