drm/i915/gem: Convert vm idr to xarray

Replace the vm_idr + vm_idr_mutex to an XArray. The XArray data
structure is now used to implement IDRs, and provides its own locking.
We can simply remove the IDR wrapper and in the process also remove our
extra mutex.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200122161531.508903-1-chris@chris-wilson.co.uk