drm: add a helper function to extract 'de-active' and 'pixelclk-active' from DT

add a helper function to extract information about pixel clock and DE
polarity from DT for use by of_get_drm_display_mode().
While at it, convert spaces to tabs in indentation in drm_modes.h.

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