drm/i915/dsb: Introduce intel_dsb_finish()

Introduce a function to emits whatever commands we need
at the end of the DSB command buffer. For the moment we
only do the tail cacheline alignment there, but eventually
we might want to eg. emit an interrupt.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118163040.29808-5-ville.syrjala@linux.intel.com
Reviewed-by: Animesh Manna <animesh.manna@intel.com>