drm/i915: GMBUS don't need no forcewake

drm/i915: GMBUS don't need no forcewake

GMBUS is part of the display engine, and thus has no need for
forcewake. Let's not bother trying to grab it then.

I don't recall if the display engine suffers from system hangs
due to multiple accesses to the same "cacheline" in mmio space.
I hope not since we're no longer protected by the uncore lock
since commit 4e6c2d58ba86 ("drm/i915: Take forcewake once for
the entire GMBUS transaction")

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Weinehall <david.weinehall@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1476272687-15070-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>