i965: Allow fast depth clears if scissoring doesn't do anything.

i965: Allow fast depth clears if scissoring doesn't do anything.

A game we're working with leaves scissoring enabled, but frequently sets
the scissor rectangle to the size of the whole screen.  In that case,
scissoring has no effect, so it's safe to go ahead with a fast clear.

Chad believe this should help with Oliver McFadden's "Dante" as well.

v2/Chad: Use the drawbuffer dimensions rather than the miptree slice
dimensions.  The miptree slice may be slightly larger due to alignment
restrictions.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-and-tested-by: Oliver McFadden <oliver.mcfadden@linux.intel.com>