drm/i915: fix for WaDisableDopClockGating:bdw

This workaround for BDW was incomplete as it also requires EUTC clock
gating to be disabled via UCGCTL1.

v2: read modify write UCGTL1 in broadwell_init_clock_gating (Ville)

Signed-off-by: Robert Bragg <robert@sixbynine.org>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170212133252.20990-1-robert@sixbynine.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>