drm/i915: move I915_IDLE_ENGINES_TIMEOUT next to its only user

Declutter i915_drv.h. If there's ever a need to use this in more than
one place, we can figure out a better spot then. For now, this seems
easiest.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230118131538.3558599-2-jani.nikula@intel.com