drm: Fix crash on imx6 when no current encoder+crtc is bound

imx6 crahes when trying drm backend. This depends heavily on
board used and this fix is generic doesn't affect existing configs
but just fixed edge case for specific board

platform/drm/cog-platform-drm.c

@@ -352,6 +352,22 @@ check_drm(void)
     return supported;
 }
 
+static int32_t
+find_crtc_for_encoder(const drmModeRes *resources, const drmModeEncoder *encoder) {
+    int i;
+
+    for (i = 0; i < resources->count_crtcs; i++) {
+        const uint32_t crtc_mask = 1 << i;
+        const uint32_t crtc_id = resources->crtcs[i];
+        if (encoder->possible_crtcs & crtc_mask) {
+            return crtc_id;
+        }
+    }
+
+    /* no match found */
+    return -1;
+}
+
 static gboolean
 init_drm(void)
 {
@@ -494,19 +510,30 @@ init_drm(void)
             (long)((drm_data.mode - drm_data.connector.obj->modes) / sizeof(drmModeModeInfo *)), drm_data.mode->name,
             drm_data.mode->vrefresh);
 
+    /* Try the currently connected encoder+crtc */
     for (int i = 0; i < drm_data.base_resources->count_encoders; ++i) {
         drm_data.encoder = drmModeGetEncoder(drm_data.fd, drm_data.base_resources->encoders[i]);
-        if (drm_data.encoder->encoder_id == drm_data.connector.obj->encoder_id)
+        if (!drm_data.encoder) {
+            /* cannot retrieve encoder, ignoring... */
+            continue;
+        }
+
+        const int32_t crtc_id = find_crtc_for_encoder(drm_data.base_resources, drm_data.encoder);
+        if (crtc_id != 0) {
+            drm_data.crtc.obj_id = crtc_id;
             break;
+        }
 
         g_clear_pointer (&drm_data.encoder, drmModeFreeEncoder);
     }
-    if (!drm_data.encoder)
+
+    if (!drm_data.encoder) {
+        fprintf(stderr, "no crtc for encoder found!\n");
         return FALSE;
+    }
 
     drm_data.connector.obj_id = drm_data.connector.obj->connector_id;
 
-    drm_data.crtc.obj_id = drm_data.encoder->crtc_id;
     drm_data.crtc.obj = drmModeGetCrtc (drm_data.fd, drm_data.crtc.obj_id);
     for (int i = 0; i < drm_data.base_resources->count_crtcs; ++i) {
         if (drm_data.base_resources->crtcs[i] == drm_data.crtc.obj_id) {