drm/i915: Disable GMBUS clock gating around GMBUS transfers on gen9+

Gen9+ need to disable GMBUS clock gating when doing multi part
transfers. Otherwise clock gating will kick in when GMBUS is in
the WAIT state and presumably that will corrupt the transfer.
This is documented as Display WA #0868.

Apparently older hardware doesn't allow clock gating in the WAIT
state and thus are unaffected by this problem.

v2: Limit the PCH w/a to gen9 and gen10 only (DK)
    Actually change it to check the PCH type instead since
    it's the PCH that actually contains the GMBUS hardware

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com> #v1
Link: https://patchwork.freedesktop.org/patch/msgid/20171221202432.17373-1-ville.syrjala@linux.intel.com