OMAP: DSS2: DSI: Get number of DSI data lanes using DSI_GNQ register

On OMAP3, the DSI module has 2 data lanes. On OMAP4, DSI1 has 4 data lanes
and DSI2 has 2 data lanes. Introduce function dsi_get_num_data_lanes() which
returns the number of data lanes on the dsi interface, introduce function
dsi_get_num_data_lanes_dssdev() which returns the number of data lanes used by
the omap_dss_device connected to the lanes.

Use the DSI_GNQ register on OMAP4 to get the number of data lanes, modify
dsi.c to use the number of lanes and the extra data lanes on DSI1.

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