drm/i915/dsb: Compute use_dsb earlier

drm/i915/dsb: Compute use_dsb earlier

Skip all the commit completion interrupt stuff on the
chained DSB when we don't take the full DSB path (ie. when
the plane/pipe programming is done via MMIO). The commit
completion will be done via the CPU side vblank interrupt.

Currently this is just a redundant interrupt, so not a big
deal. But in the future we'll be moving the TRANS_PUSH write
into the chained DSB as well, and that we definitely don't
want to do when it's also being done by the CPU from
intel_pipe_update_end().

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250207223159.14132-5-ville.syrjala@linux.intel.com
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>