drm/i915/dsb: Introduce intel_dsb_exec_time_us()

Pull the magic 20 usec DSB execution deadline into
intel_dsb_arm_exec_time_us(), and also add its counterpart
for the non-arming register write section. For the non-arming
part we'll just throw in a random 80 usec for now so the total
is 100usec. The total exec time will be needed by the upcoming
flip queue code.

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/20250612145018.8735-4-ville.syrjala@linux.intel.com