drm/vmwgfx: Use a per-device semaphore for reservation protection

Don't use a per-master semaphore (ttm lock) for reservation protection, but
rather a per-device semaphore. This is needed since clients connecting using
render nodes aren't master aware.

The ttm lock used should probably be replaced with a reader-write semaphore
once the function down_xx_interruptible() is available.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>