drm/i915: s/PUNIT_REG_DSPFREQ/PUNIT_REG_DSPSSPM/

Rename the punit display power register to match the spec.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181129175504.3630-1-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>