drm/i915: Enable HPLL watermarks on g4x

drm/i915: Enable HPLL watermarks on g4x

I don't see why we couldn't use the HPLL watermarks on g4x. So let's
enable them. Let's assume a 35 usec memory latency for the HPLL mode.
That's roughly what PNV uses.

Based on the behaviour of the ELK box I have 35 usec is probably
overkill. Actually all the current latency values used seem overkill as
I can reduce them pretty drastically before I start to see underruns.
But let's play things a bit safe for now.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170421181432.15216-14-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>