drm: cleanup buffer/map code

This is a patch from DRM CVS that cleans up some code that was in CVS
that I never moved to the kernel, this patch produces the result of the
cleanups and puts it into the kernel drm.

From: Eric Anholt <anholt@freebsd.org>, Jon Smirl, Dave Airlie
Signed-off-by: Dave Airlie <airlied@linux.ie>