drm/i915/psr: Avoid initializing PSR if there is no sink support.

DPCD read for the eDP is complete by the time intel_psr_init() is
called, which means we can avoid initializing PSR structures and state
if there is no sink support.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180103213824.1405-3-dhinakaran.pandiyan@intel.com