drm/i915/dsb: hide struct intel_dsb better

struct intel_dsb can be an opaque type, hidden in intel_dsb.c. Make it
so. Reduce related includes while at it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908165702.973854-1-jani.nikula@intel.com