OMAP: DSS2: DSI: Get line buffer size from DSI_GNQ register

The line buffer sizes vary across DSI modules, create a function
dsi_get_line_buf_size() using DSI_GNQ register to get the size of
line buffer used for the DISPC video port data.

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