drm/i915/dsi: remove unused _MIPIA_AUTOPWG register definition

There are other unused registers, but this is also unusable and
inadequate. Remove.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/57afda02856a68f78fe4d30384d4f7b352b9a812.1713520813.git.jani.nikula@intel.com


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