drm: drop setting vm_file to filp

Talking to Al Viro on irc, we can see no possible reason for doing
this, the upper mmap code does it. The code has been there since
first import into drm tree I can find.

Al tracked down this as a requirement pre 2.3.51 hasn't been needed since.

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>