RFC drm: add support for for clk and de polarity & gpu: ipu-v3: use clock and de polarity

Hello!

These patches address a problem we ran into using parallel displays with Freescale i.MX53 and i.MX6 SoC's.
In short: We wanted to change the clock signal polarity by using display-timing in the devicetree description, but the output signal stayed unchanged.

Parallel displays may have different polarities for clock and data enable signals. (Clock polarity is also a topic on LVDS displays). This is the reason why there are some properties in devicetree display-timing [1] called
* pixelclk-active .. pixel clock polarity
* de-active .. data enable pulse polarity

This properties are correctly represented in struct display_timings [2] and struct videomode [3] by using enum display_flags [3]. But when it comes to struct drm_display_mode [4] there are no representations for this.
The properties are "lost in conversion" and never reach imx drm. (or other drm drivers).

Changing this would be especially important for embedded devices where parallel(RGB) and LVDS displays are still widely used and drm already plays an important role.

Following two patches will
1. Introduce representation of clock and data enable polarities in struct drm_display_mode analog to hsync/vsync signals.
* "drm: add support for for clk and de polarity"
2. Implicitly enable usage of the newly introduced flags in imx drm (with respect to devicetree compatibility.)
* "gpu: ipu-v3: use clock and de polarity from videomode"

I'm aware that introducing new flags in struct drm_display_mode affects user space too and therefore has to be considered carefully. As I'm not really deep in drm user space your suggestions are very welcome.

best regards,
manfred

[1] Documentation/devicetree/bindings/video/display-timing.txt
[2] include/video/display_timing.h
[3] include/video/videomode.h
[4] include/drm/drm_modes.h