lib/igt_psr: Move "is enabled" implementation to lib

i915_pm_dc have implemented a function to check if PSR2 is enabled,
that may be util to future tests so move it to PSR lib.

Cc: Jeevan B <jeevan.b@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>