drm/edid: use struct detailed_timing member access in is_rb()

Use struct detailed_timing member access instead of direct offsets to
avoid casting.

Use BUILD_BUG_ON() for sanity check.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/c069669c2fe8f9c3061c7d1a413c75a33ec48813.1648477901.git.jani.nikula@intel.com

drivers/gpu/drm/drm_edid.c

@@ -2408,15 +2408,19 @@ drm_for_each_detailed_block(u8 *raw_edid, detailed_cb *cb, void *closure)
 }
 
 static void
-is_rb(struct detailed_timing *t, void *data)
+is_rb(struct detailed_timing *descriptor, void *data)
 {
-	u8 *r = (u8 *)t;
+	bool *res = data;
 
-	if (!is_display_descriptor(t, EDID_DETAIL_MONITOR_RANGE))
+	if (!is_display_descriptor(descriptor, EDID_DETAIL_MONITOR_RANGE))
 		return;
 
-	if (r[10] == DRM_EDID_CVT_SUPPORT_FLAG && r[15] & 0x10)
-		*(bool *)data = true;
+	BUILD_BUG_ON(offsetof(typeof(*descriptor), data.other_data.data.range.flags) != 10);
+	BUILD_BUG_ON(offsetof(typeof(*descriptor), data.other_data.data.range.formula.cvt.flags) != 15);
+
+	if (descriptor->data.other_data.data.range.flags == DRM_EDID_CVT_SUPPORT_FLAG &&
+	    descriptor->data.other_data.data.range.formula.cvt.flags & 0x10)
+		*res = true;
 }
 
 /* EDID 1.4 defines this explicitly.  For EDID 1.3, we guess, badly. */