intel: Assert that no batch is emitted if a region is mapped

intel: Assert that no batch is emitted if a region is mapped

What I would prefer to assert is that, for each region that is currently
mapped, no batch is emitted that uses that region's bo. However, it's much
easier to implement this big hammer.

Observe that this requires that the batch flush in intel_region_map() be
moved to within the map_refcount guard.

v2: Add comments (borrowed from anholt's reply) explaining why the
assertion is a good idea.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Chad Versace <chad@chad-versace.us>