drm/imx: convey the pixelclk-active and de-active flags from DT to the ipu-di driver

The 'de-active' and 'pixelclk-active' DT properties are evaluated
by of_parse_display_timing() called from  of_get_drm_display_mode(),
but later lost in the conversion from videomode.flags to
drm_display_mode.flags.
Enhance of_get_drm_display_mode() to also return the bus flags in a
separate variable, so that they can be passed on to the ipu-di
driver.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>