drm/omap: move out of staging

Now that the omapdss interface has been reworked so that omapdrm can use
dispc directly, we have been able to fix the remaining functional kms
issues with omapdrm.  And in the mean time the PM sequencing and many
other of that open issues have been solved.  So I think it makes sense
to finally move omapdrm out of staging.

Signed-off-by: Rob Clark <robdclark@gmail.com>