drm/omap: remove set but not used variable 'frame_height'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/omapdrm/dss/dispc.c: In function 'dispc_ovl_setup_common':
drivers/gpu/drm/omapdrm/dss/dispc.c:2627:19: warning:
 variable 'frame_height' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>