drm/i915/dsb: Use intel_dsb_ins_align() in intel_dsb_align_tail()

If the free_post is not QW aligned we don't have to memset the
extra DW needed to make it so, as the only way that can happen
is via intel_dsb_reg_write_indexed() which already makes sure
the next DW is zeroed.

Not a big deal, but this is more consistent how all the other
stuff operates that puts instructions into the DSB buffer, and
we'll get a few more of those soon.

Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250612145018.8735-2-ville.syrjala@linux.intel.com