drm/mm: include required headers in drm_mm.h

We need BUG_ON(), spinlock_t and standard kernel data-types so include the
right headers.

Subject: [drm-intel:drm-intel-nightly 154/166] include/drm/drm_mm.h:67:2:
 error: unknown type name 'spinlock_t'
Message-ID: <51f14693.g5HGdcuw2v3m8FOd%fengguang.wu@intel.com>

In case it didn't link to it correctly. Somehow this bug doesn't occur
here on my machine, hmm. But I think fixing drm_mm.h is better than
changing the include-order in drm_vma_manager.h, so this is what I
did.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>