drm: fix a LOR issue on FreeBSD for savage driver

Correct a LOR issue on FreeBSD by allocating temporary space and doing a single
DRM_COPY_FROM_USER rather than DRM_VERIFYAREA_READ followed by tons of
DRM_COPY_FROM_USER_UNCHECKED.  I don't like the look of the temporary space
allocation, but I like the simplification in the rest of the file.  Tested
with glxgears, tuxracer, and q3 on a savage4.

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