tests/i915/i915_suspend: Disable d3cold_allowed for basic-s2idle-without-i915

When module is unloaded and s2idle is triggered,
PCI core leaves the endpoint in D0 and the bridge in D3 state causing
PCIE spec violation. The config space is read as 0xFF

Keep the bridge in D0 before module unload to prevent
this issue

v2: Addressed cosmetic review comments (Anshuman)

Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Reviewed-by: Anshuman Gupta <anshuman.gupta@intel.com>