drm/i915/dsb: Extract intel_dsb_emit()

Extract a small helper to emit a DSB intstruction. Should
become useful if/when we need to start emitting other
instructions besides register writes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-7-ville.syrjala@linux.intel.com


Reviewed-by: Animesh Manna <animesh.manna@intel.com>