drm/vmwgfx: Convert to new IDA API

Reorder allocation to avoid an awkward lock/unlock/lock sequence.
Simpler code due to being able to use ida_alloc_max(), even if we can't
eliminate the driver's spinlock.

Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>