drm: Add support for drm master_[set|drop] callbacks.

The vmwgfx driver has a per master rw lock around TTM, to guarantee 
mutual exclusion when needed.

This is typically when all evictable buffers are evicted due to

1) vt switch
2) master switch
3) suspend / resume.

In the multi-master case, on master switch the new master takes the 
previously active master lock in write mode, and then evicts all 
buffers. Any clients to previous masters will then block on that lock 
when trying to validate a buffer. fbdev also acts as a virtual master
wrt this.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>