drm/i915/scaler: Remove redudant junk from skl_scaler.h

Nuke some redundant includes and forward declarations from
skl_scaler.h.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241107122658.21901-3-ville.syrjala@linux.intel.com