drm/i915/phys: unconditionally call release_memory_region

The release method will undo what we did at creation, and so we
shouldn't care if we have pages or not. Fixes a small leak in the
mock_phys selftest.

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305204258.216302-1-matthew.auld@intel.com