This is a port of Jason's 8379bff6
from i965 to iris.  We can't find anything relevant in the documentation
and no one we've talked to has been able to help us pin down a solution.

Unfortunately, we have to put the hack in both iris_blit() and
iris_copy_region().  st/mesa's CopyImage() implementation sometimes
chooses to use pipe->blit() instead of pipe->resource_copy_region().
For blits, we only do the hack if the blit source format doesn't match
the underlying resource (i.e. it's reinterpreting the bits).  Hopefully
this should not be too common.