OMAP: DSS2: Add method for querying display dimensions from DSS drivers

Add get_dimensions() to struct omap_dss_driver. Use the call, if supported
by the driver, in OMAPFB.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>