glamo_fb: Implement screen blanking

This patch implements fb_blank() for the glamo-fb driver, which switches off
the pixel clock (DCLK) for power saving.

We currently delay the actual pixel clock switch until we enter
FB_BLANK_POWERDOWN, since the backlight fade is slow and we don't want the
user to see artefacts on the screen while the backlight is fading out.

So since the X server first sends FB_BLANK_{V,H}SYNC_SUSPEND, we start the
backlight fade here, and only once we get FB_BLANK_POWERDOWN the pixel clock is
disabled.

There are no measurements yet, but the power savings should be double, since
there is no longer any  generation of the high-frequency LCM signals, and
there are no video-related SDRAM accesses anymore.

Signed-off-by: Harald Welte <laforge@openmoko.org>

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