drm/i915/skl: Disable coarse power gating up until F0

drm/i915/skl: Disable coarse power gating up until F0

There is conflicting info between E0 and F0 steppings
for this workarounds. Trust more authoritative source and
be conservative and extend also for F0.

This prevents numerous (>50) gpu hangs with SKL GT4e
during piglit run.

References: HSD: gen9lp/2134184
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1449505785-20812-1-git-send-email-mika.kuoppala@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>