OMAP: DSS2: Use a macro to declare size of the fifo_size array in dispc.c

The array size of fifo_size array in the global dispc struct is currently
hardcoded to 3. Replace this with the MAX_DSS_OVERLAYS macro in dss_features.h,
use dss_features function to get the number of overlays instead of the
ARRAY_SIZE macro in dispc_read_plane_fifo_sizes().

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