drm/i915/dsb: move i915 specific DSB buffer implementation to i915
The DSB buffer implementation is different for both i915 and xe. Move
the i915 specific implementation from display to i915 core.
Reviewed-by: Michał Grzelak <michal.grzelak@intel.com>
Link: https://patch.msgid.link/a01b3e69a3f79713eacf437af00dbe23d2f9c8dd.1768923917.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>