drm/nouveau: Fix nouveau_connector_ddc_detect()

It looks like that when we moved over to using
drm_connector_for_each_possible_encoder() in nouveau, that one rather
important part of this function got dropped by accident:

	/*          Right   v   here */
	for (i = 0; nv_encoder = NULL, i < DRM_CONNECTOR_MAX_ENCODER; i++) {
		int id = connector->encoder_ids[i];
		if (id == 0)
			break;

Since it's rather difficult to notice: the conditional in this loop is
actually:

	nv_encoder = NULL, i < DRM_CONNECTOR_MAX_ENCODER

Meaning that all early breaks result in nv_encoder keeping it's value,
otherwise nv_encoder = NULL. Ugh.

Since this got dropped, nouveau_connector_ddc_detect() now returns an
encoder for every single connector, regardless of whether or not it's
detected:

    [ 1780.056185] nouveau 0000:01:00.0: DRM: DDC responded, but no EDID for DP-2

So: fix this to ensure we only return an encoder if we actually found
one, and clean up the rest of the function while we're at it since it's
nearly impossible to read properly.

Changes since v1:
- Don't skip ddc probing for LVDS if we can't switch DDC through
  vga-switcheroo, just do the DDC probing without calling
  vga_switcheroo_lock_ddc() - skeggsb

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: ddba766d ("drm/nouveau: Use drm_connector_for_each_possible_encoder()")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>