drm/i915/plane: rename intel_atomic_plane.[ch] to intel_plane.[ch]

It's all atomic, no need to emphasize this.

v2: Also update Documentation/gpu/i915.rst (Gustavo)

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/ba5f304e9fe71723191d872e6828d461e1a572bd.1750147992.git.jani.nikula@intel.com


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