drm: Pass pointers to virt_to_page()

Most architectures define virt_to_page() as a macro that casts its
argument such that an argument of type unsigned long will be accepted
without complaint.  However, the proper type is void *, and passing
unsigned long results in a warning on MIPS.

Compile-tested only.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>