drm/i915: Rename i915_{save,restore}_state()

drm/i915: Rename i915_{save,restore}_state()

i915_{save,restore}_state() are actually all about the display.
Currently they are split into display part + SWF part. But since
the SWF part is also related to the display let's just move that
part into its own thing and flip the roles around so that the
current display part is the main function.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201005171441.26612-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>