drm/i915: Add way to specify the power-off delay of a display power domain

Add support for specifying a delay different than the current 100 ms
default for powering off a display power domain. This is needed by the
next patch which delays re-enabling DC states during modesets to avoid
the off->on->off toggling overhead of the DC_off power well, but does
this using a < 100 ms delay for a better utilization of DC power saving
states.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616185104.2502003-3-imre.deak@intel.com