drm/i915: split out i9xx_wm_regs.h
Very few files need the i9xx watermark related registers. Split them out
to a dedicated file.
Reviewed-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241213115111.335474-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>