drm/i915: Clean up the PNV bit banging vs. GMBUS clock gating w/a

Give a proper name for the GMBUS clock gating disable bit on PNV,
and rename intel_i2c_quirk_set() to pnv_gmbus_clock_gating() for
clarity.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171208213739.16388-3-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>