drm/i915/psr: Add new define for PSR idle timeout

Currently we are using value 50ms as timeout for waiting PSR to idle. Add
own define for this purpose.

Signed-off-by: Jouni Högander <jouni.hogander@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://lore.kernel.org/r/20250905072708.2659411-3-jouni.hogander@intel.com