drm/i915: Tighten SAGV constraint for pre-tgl

Say we have two planes enabled with watermarks configured
as follows:
plane A: wm0=enabled/can_sagv=false, wm1=enabled/can_sagv=true
plane B: wm0=enabled/can_sagv=true,  wm1=disabled

This is possible since the latency we use to calculate
can_sagv may not be the same for both planes due to
skl_needs_memory_bw_wa().

In this case skl_crtc_can_enable_sagv() will see that
both planes have enabled at least one watermark level
with can_sagv==true, and thus proceeds to allow SAGV.
However, since plane B does not have wm1 enabled
plane A can't actually use it either. Thus we are
now running with SAGV enabled, but plane A can't
actually tolerate the extra latency it imposes.

To remedy this only allow SAGV on if the highest common
enabled watermark level for all active planes can tolerate
the extra SAGV latency.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210305153610.12177-3-ville.syrjala@linux.intel.com
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>

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