drm/i915/dsi: remove dependency on i915_drv.h

Remove final references to struct drm_i915_private and drop dependency
on i915_drv.h.

Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/2cee3cd9d7d9bec8dfe9c21fe5d172b1843b3d97.1746529001.git.jani.nikula@intel.com


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