drm/i915/psr: avoid intel_frontbuffer.h include with declaration

Reduce include dependencies using forward declarations.

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/ee35f2f01f731c21f24a238f2d1690b09ac2da1f.1639142167.git.jani.nikula@intel.com