i965: Enable fast clears for multi-lod

i965: Enable fast clears for multi-lod

On SKL (also fast clear is used for level 0, layer 0):

Manhattan 3.0:          3.88434% +/- 0.814659%
Manhattan 3.0 off:      3.25542% +/- 0.101149%
Trex:                   3.43501% +/- 0.31223%
Trex off:               4.13781% +/- 0.0993569%

ON BDW:

Manhattan 3.0:          1.37079% +/- 0.571208%
Manhattan 3.0 off:      1.74029% +/- 0.267499%

v2 (Ben, Matt): Fix rebase error by removing the perf warning
v3 (Topi): Rebased on top of revised eligibility logic

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>