drm/i915: Move function prototypes to the correct header

A bunch of function prototypes were left behind when the
plane/crtc code got reshuffled to new files. Move the
prototypes as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211020223339.669-2-ville.syrjala@linux.intel.com


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