drm/i915/ddi: Guard reg_val against a INVALID_TRANSCODER

Currently we check if the encoder is INVALID or -1 and throw a
WARN_ON but we still end up writing the temp value which will
overflow and corrupt the whole programmed value.

--v2
-Assign a bogus transcoder to master in case we get a INVALID
TRANSCODER [Jani]

Fixes: 6671c36 ("drm/i915/tgl: Select master transcoder for MST stream")
Signed-off-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20250908042208.1011144-1-suraj.kandpal@intel.com
(cherry picked from commit c8e8e9a)
Signed-off-by: Tvrtko Ursulin <tursulin@ursulin.net>

Author: surajk8

drivers/gpu/drm/i915/display/intel_ddi.c

@@ -596,8 +596,9 @@ intel_ddi_transcoder_func_reg_val_get(struct intel_encoder *encoder,
 			enum transcoder master;
 
 			master = crtc_state->mst_master_transcoder;
-			drm_WARN_ON(display->drm,
-				    master == INVALID_TRANSCODER);
+			if (drm_WARN_ON(display->drm,
+					master == INVALID_TRANSCODER))
+				master = TRANSCODER_A;
 			temp |= TRANS_DDI_MST_TRANSPORT_SELECT(master);
 		}
 	} else {