drm: apple: Explicitly set identity matrix when CTM blob is not set

Unset CTM means a unit/pass-thru matrix should be used. Since we were
checking for a valid CTM blob, this was resulting in a misconfigured
colour management pipeline.
This was most noticeable when using kwin's Night Light feature, specifically
when waking the display after the morning transition was supposed to
happen. Despite triggering a colour management change on CRTC enable,
kwin clearing the CTM blob meant that there was nothing for us to passs
in to IOMFB to set DCP's internal CTM.

Explicitly pass the identity matrix to IOMFB if we have a pending
colour management change and no CTM blob.

Signed-off-by: James Calligeros <jcalligeros99@gmail.com>
Signed-off-by: Janne Grunau <j@jannau.net>

---
- simplify after refactoring and fix value for 1 (S31.32 sign-magnitude)
- adjusted commit message to documented DRM KMS behaviour

Author: chadmed

drivers/gpu/drm/apple/iomfb_template.c

@@ -1406,12 +1406,18 @@ void DCP_FW_NAME(iomfb_flush)(struct apple_dcp *dcp, struct drm_crtc *crtc, stru
 		dcp->brightness.update = false;
 	}
 
-	if (crtc_state->color_mgmt_changed && crtc_state->ctm) {
-		struct drm_color_ctm *ctm = (struct drm_color_ctm *)crtc_state->ctm->data;
+	if (crtc_state->color_mgmt_changed) {
 		struct iomfb_set_matrix_req mat = {
 			.location = 9,
 		};
-		memcpy(mat.matrix, ctm->matrix, sizeof(mat.matrix));
+
+		if (crtc_state->ctm) {
+			struct drm_color_ctm *ctm = (struct drm_color_ctm *)crtc_state->ctm->data;
+			memcpy(mat.matrix, ctm->matrix, sizeof(mat.matrix));
+		} else {
+			mat.matrix[0] = mat.matrix[4] = mat.matrix[8] = 1LLU << 32;
+		}
+
 		iomfb_set_matrix(dcp, false, &mat, do_swap, NULL);
 	} else
 		do_swap(dcp, NULL, NULL);