i965: Add gen8 fast clear perf debug

i965: Add gen8 fast clear perf debug

In an ideal world I would just implement this instead of adding the perf debug.
There are some errata involved which lead me to believe it won't be so simple as
flipping a few bits.

There is room to add a thing for Gen9s flexibility, but since I am actively
working on that I have opted to ignore it.

Example:
Multi-LOD fast clear - giving up (256x128x8).

v2: Use braces for if statements because they are multiple lines (Ken)

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>