drm/i915: implement a media hang w/a

Contrary to the other clock gating w/a in GEN6_UCGCTL1, this one is
actually documented in Bspec, vol1g "GT Interface Registers [SNB]",
Section 1.5.1 "UCGCTL1 - Unit Level Clock Gating Control 1".

Supposedly this can prevent hangs on the media ring.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>