omap: add omapdrm support

This adds libdrm_omap helper layer (as used by xf86-video-omap,
omapdrmtest, etc).

Signed-off-by: Rob Clark <rob@ti.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
[danvet: pushed for Rob, he doesn't yet have commit access.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>