drm/i915: Drop a useless forward declararion

ilk_pfit_enable() is defined before the first use. No need
for a forwared declaration.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230418175528.13117-9-ville.syrjala@linux.intel.com


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