drm/i915: Rename skl_wa_clkgating to the actual WA

No functional change. Renaming the function to reflect the specific WA.

Suggested-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190330011921.10397-1-radhakrishna.sripada@intel.com