drm/i915/dsb: Extract assert_dsb_tail_is_aligned()

drm/i915/dsb: Extract assert_dsb_tail_is_aligned()

Extract the DSB tail alignment checks into helper. We already
have two uses of this, and soon we'll get a third.

v2: s/soo/soon in commit message (Animesh)

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/20250523062041.166468-3-chaitanya.kumar.borah@intel.com