Use the igt_set_module_param_int() call to enable it, then restore the
previous value after we are done testing.

With this, we can change the psr_enabled() function to psr_possible():
the only requirement should be that we have a PSR capable sink. The
test should now be able to make "Source_OK" and "Enabled" become true
whenever it wants.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>