i965: Prevent fast clears for MSRTs on SKL

There are currently a bunch of formats that behave strangely when
sampling the cleared color from the MCS buffer on SKL. They seem to
mostly be formats that don't have an alpha component, although it's
not all of them, and we haven't yet found anything in the specs which
would explain this. For now to be on the safe side this patch just
prevents fast clears for MSRTs on SKL altogether so that when fast
clears are eventually enabled it will only be for single-sampled
surfaces. The assumption is that clears are probably more likely to be
used in single-sampled applications anyway so we can at least get them
working and we can enable MSRTs later once we understand the problem
better.

This patch should have no functional effect other than perhaps
receiving fewer perf_debug messages on SKL+.

v2: Improve the commit message to avoid saying the patch disables fast
    clears because it will be merged before fast clears are enabled
    for any surfaces so it doesn't actually disable anything.
Reviewed-by: Ben Widawsky <[email protected]>
Reviewed-by: Chad Versace <[email protected]>

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