OMAPDSS: Optionally enable write-through cache for the framebuffer

Write-through cached framebuffer eliminates the need for using shadow
framebuffer in xf86-video-fbdev DDX. At the very least this reduces
memory footprint, but the performance is also the same or better
when moving windows or scrolling on ARM11 and Cortex-A8 hardware.

Benchmark with xf86-video-fbdev on IGEPv2 board (TI DM3730, 1GHz),
1280x1024 screen resolution and 32bpp desktop color depth:

$ x11perf -scroll500 -copywinwin500 -copypixpix500 \
          -copypixwin500 -copywinpix500

-- omapfb.vram_cache=n, Option "ShadowFB" "true" in xorg.conf
 10000 trep @   3.4583 msec ( 289.0/sec): Scroll 500x500 pixels
  6000 trep @   4.3255 msec ( 231.0/sec): Copy 500x500 from window to window
  8000 trep @   3.2738 msec ( 305.0/sec): Copy 500x500 from pixmap to window
  8000 trep @   3.1707 msec ( 315.0/sec): Copy 500x500 from window to pixmap
  8000 trep @   3.4761 msec ( 288.0/sec): Copy 500x500 from pixmap to pixmap

-- omapfb.vram_cache=n, Option "ShadowFB" "false" in xorg.conf
  5000 trep @   5.2357 msec ( 191.0/sec): Scroll 500x500 pixels
  1200 trep @  21.0346 msec (  47.5/sec): Copy 500x500 from window to window
  8000 trep @   3.1590 msec ( 317.0/sec): Copy 500x500 from pixmap to window
  6000 trep @   4.5062 msec ( 222.0/sec): Copy 500x500 from window to pixmap
  8000 trep @   3.4767 msec ( 288.0/sec): Copy 500x500 from pixmap to pixmap

-- omapfb.vram_cache=y, Option "ShadowFB" "true" in xorg.conf
 10000 trep @   3.4580 msec ( 289.0/sec): Scroll 500x500 pixels
  6000 trep @   4.3424 msec ( 230.0/sec): Copy 500x500 from window to window
  8000 trep @   3.2673 msec ( 306.0/sec): Copy 500x500 from pixmap to window
  8000 trep @   3.1626 msec ( 316.0/sec): Copy 500x500 from window to pixmap
  8000 trep @   3.4733 msec ( 288.0/sec): Copy 500x500 from pixmap to pixmap

-- omapfb.vram_cache=y, Option "ShadowFB" "false" in xorg.conf
 10000 trep @   3.4893 msec ( 287.0/sec): Scroll 500x500 pixels
  8000 trep @   4.0600 msec ( 246.0/sec): Copy 500x500 from window to window
  8000 trep @   3.1565 msec ( 317.0/sec): Copy 500x500 from pixmap to window
  8000 trep @   3.1373 msec ( 319.0/sec): Copy 500x500 from window to pixmap
  8000 trep @   3.4631 msec ( 289.0/sec): Copy 500x500 from pixmap to pixmap

[notasas@gmail.com: enabled it by default, changed perms]
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
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