I have various conditions in place to try and avoid unnecessary
PIPE_CONTROL flushes, especially to batches which may have never
used the buffer being mapped.  But if we do a CPU map to a bound
constant buffer, we still need to mark push constants dirty, even
if there's nothing happening in batches that would warrant a flush.

Fixes obvious misrendering in the "XCOM 2: War of the Chosen" menus
(lots of rainbow colored triangles).  Fixes lots of blinking elements
in "Shadow of Mordor".  Fixes missing crowd rendering in "DiRT Rally".