drm/i915: remove g4x lowfreq_avail and has_pipe_cxsr

They're unused and unsupported. Leave the reduced_clock pointers in
place still, should they prove useful later on.

v2: go from nuking DDI lowfreq_avail to nuking it entirely (Ville)

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Suggested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171017140234.20677-1-jani.nikula@intel.com