drm/i915/pfit: Extract intel_pfit.c

The panel fitter code doesn't really have much to do with the
rest of intel_panel.c, so extract it all into its own file.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241016143134.26903-9-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>