drm: Make sure the DRM offset matches the CPU

The pgoff option in mmap() is defined as an unsigned long
so the offset generated by DRM needs to fit into
BITS_PER_LONG for the CPU in question.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>