drm/i915/dsb: Extract intel_dsb_ins_align()

drm/i915/dsb: Extract intel_dsb_ins_align()

Extract the code that alings the next instruction to the next
QW boundary into a small helper. I'll have some more uses for
this later.

Also explain why we don't have to zero out the extra DW.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://lore.kernel.org/r/20250523062041.166468-2-chaitanya.kumar.borah@intel.com