drm/i915/plane: make intel_plane_atomic_check() static and rename

intel_plane_atomic_check() isn't used outside of intel_plane.c. Make it
static. While at it, rename to vacate the name for subsequent changes.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/9da965c23c1485625d8713152751470ee758d540.1750147992.git.jani.nikula@intel.com


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