drm/vmwgfx: Remove the resource avail field

This field was previously used to prevent a lookup of a resource before its
constructor had run to its end. This was mainly intended for an interface
that is now removed that allowed looking up a resource by its device id.

Currently all affected resources are added to the lookup mechanism (its
TTM prime object is initialized) late in the constructor where it's OK to
look up the resource.

This means we can change the device resource_lock to an ordinary spinlock
instead of an rwlock and remove a locking sequence during lookup.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Deepak Rawat <drawat@vmware.com>