drm/i915: Prevent needless toggling of DC states during modesets

DC states are disabled / re-enabled around each modeset, which may lead
to a needless off->on->off toggling of the DC_off power well. This has
some overhead as toggling DC states involves running a DMC firmware
handler and also running a periodic firmware handler while DC states are
enabled. The limit of when DC states have a benefit is at 30 FPS (using
DC3co) and below 30 FPS (using DC5/6), where the firmware can actually
disable clocks / power off power wells. Accordingly delay powering off
the DC_off powerwell (which re-enables DC states) by 17 ms at the end of
a modeset to avoid the above overhead at or above 60 FPS.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616185104.2502003-4-imre.deak@intel.com