drm/amd/display: Guard against null crtc in CRC IRQ

[Why]

A null pointer deference can occur if crtc is null in
amdgpu_dm_crtc_handle_crc_irq. This can happen if get_crtc_by_otg_inst
returns NULL during dm_crtc_high_irq, leading to a hang in some IGT
test cases.

[How]

Check that CRTC is non-null before accessing its fields.

Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Sun peng Li <Sunpeng.Li@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_crc.c

@@ -98,10 +98,16 @@ int amdgpu_dm_crtc_set_crc_source(struct drm_crtc *crtc, const char *src_name,
  */
 void amdgpu_dm_crtc_handle_crc_irq(struct drm_crtc *crtc)
 {
-	struct dm_crtc_state *crtc_state = to_dm_crtc_state(crtc->state);
-	struct dc_stream_state *stream_state = crtc_state->stream;
+	struct dm_crtc_state *crtc_state;
+	struct dc_stream_state *stream_state;
 	uint32_t crcs[3];
 
+	if (crtc == NULL)
+		return;
+
+	crtc_state = to_dm_crtc_state(crtc->state);
+	stream_state = crtc_state->stream;
+
 	/* Early return if CRC capture is not enabled. */
 	if (!crtc_state->crc_enabled)
 		return;