i965: Drop a funny assert about mt pitch.

i965: Drop a funny assert about mt pitch.

I slipped this in in the region->pitch change from pixels to bytes, but I
don't see any reason for it any more -- the libdrm code doesn't appear to
divide pitch by a cpp.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>