drm/i915/display: Only query DP state of a DDI encoder

drm/i915/display: Only query DP state of a DDI encoder

Avoid a NULL dereference for a mismatched encoder type, hit when
probing state for all encoders.

This is a band aid to prevent the OOPS as the right fix is "probably to
swap the psr vs infoframes.enable checks, or outright disappear from
this function" (Ville).

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1892
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525124912.16019-1-chris@chris-wilson.co.uk