OMAPDSS: RFBI: Add function to set panel size

RFBI drivers requires configuration of the update area. Since we don't support
partial updates, the size to be configures is the panel size itself.

Add a timings field in RFBI's driver data. Apart from x_res and y_res, all the
other fields are configured to an initial value when RFBI is enabled. A panel
driver is expected to call omapdss_rfbi_set_size() configure the size of the
panel.

Signed-off-by: Archit Taneja <archit@ti.com>