drm/i915/kbl: Add WaDisableGafsUnitClkGating

drm/i915/kbl: Add WaDisableGafsUnitClkGating

We need to disable clock gating in this unit to work around
hardware issue causing possible corruption/hang.

v2: name the bit (Ville)
v3: leave the fix enabled for 2227050 and set correct bit (Matthew)
v4: Split out the skl part in separate commit for easier backport

References: HSD#2227156, HSD#2227050
Cc: Ville Syrjälä <[email protected]>
Cc: Matthew Auld <[email protected]>
Signed-off-by: Mika Kuoppala <[email protected]>
Reviewed-by: Matthew Auld <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]