drm/i915/psr: Kill psr.source_ok flag.

This flag has become redundant since
commit 4d90f2d5 ("drm/i915: Start tracking PSR state in crtc state")
It is set at the same place as psr.enabled, which is also exposed via
debugfs.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180103213824.1405-1-dhinakaran.pandiyan@intel.com