drm/i915: Make active_pipes check skl specific

Seems that only skl needs to have SAGV turned off
for multipipe scenarios, so lets do it this way.

If anything blows up - we can always revert this patch.

v2: Changed if condition to look better (Ville).

Signed-off-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
[vsyrjala: wrapped long line to appease checkpatch]
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200513093816.11466-4-stanislav.lisovskiy@intel.com