drm/i915: move more atomic plane declarations to intel_atomic_plane.h

Some function declarations in intel_drv.h were missed when
intel_atomic_plane.h was created.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606122203.13416-3-jani.nikula@intel.com