drm/vmwgfx: Remove the reservation semaphore

Now since Christian reworked TTM to always keep objects on the LRU
list unless they are pinned we shouldn't need the reservation
semaphore. It makes the driver code a lot cleaner, especially
because it was a little hard to reason when and where the
reservation semaphore needed to be held.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210505035740.286923-5-zackr@vmware.com