i965/meta: Assert fast clears and rep clears never overlap

i965/meta: Assert fast clears and rep clears never overlap

There is nothing wrong with the code today, but as one modifies the code it
turns out to be not too difficult to mess up the code, and this easy assertion
should catch such driver implementation failures quickly.

Cc: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chad Versace <chad.versace@intel.com>
Reviewed-by: Neil Roberts <neil@linux.intel.com>