drm/i915/dsb: Introduce intel_dsb_align_tail()

Move the DSB tail cacheline alignment to a helper. No need to pollute
the caller with mundane details like this.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221216003810.13338-10-ville.syrjala@linux.intel.com


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