drm/i915: Implement WaDisableEarlyEOT.

There seems to be another clock gating issue which the workaround is
described as:

   "WA: Set 0xE4F0[1] = 1 to disable Early EOT of thread."

Signed-off-by: Rafael Antognolli <rafael.antognolli@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171216001117.14232-2-rafael.antognolli@intel.com