drm/vc4_hdmi: Force modeset when bcp changes

See: https://forum.libreelec.tv/thread/25427-le-10-0-2-on-rpi4-not-playing-files-that-10-0-1-had-no-problems-with/

The issue is that kodi changes hdmi mode to 3840x2160@24 initially with "max bcp=8"
After decoding the first frame it does a modeset to same mode with "max bpc=12".

Now vc4_hdmi_encoder_compute_config chooses vc4_state->output_bpc = 12 with output_format=VC4_HDMI_OUTPUT_RGB
This requires scrambling as clock > 300MHz (and we have hdmi_enable_4kp60=1).

vc4_hdmi_encoder_atomic_mode_set (without this PR's assignment to mode_changed) is currenly not called so we don't assign:
vc4_hdmi->output_bpc = vc4_state->output_bpc

which means vc4_hdmi_enable_scrambling never enables scrambling (as vc4_hdmi->output_bpc is still 8).

But we do set the pixel clock in phy_init() to a clock frequency that requires scrambling.

The inconsistency of changing pixel clock, but not changing related state that is needed to enable scambling seems wrong.
I feel this PR works around the problem, rather than fixing it, so I'd be interested in hearing how it should work.

Signed-off-by: Dom Cobley <popcornmix@gmail.com>

Author: popcornmix

drivers/gpu/drm/vc4/vc4_hdmi.c

@@ -1808,6 +1808,8 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 	struct vc4_hdmi_connector_state *vc4_state = conn_state_to_vc4_hdmi_conn_state(conn_state);
 	struct drm_display_mode *mode = &crtc_state->adjusted_mode;
 	struct vc4_hdmi *vc4_hdmi = encoder_to_vc4_hdmi(encoder);
+	unsigned int output_bpc = vc4_state->output_bpc;
+	enum vc4_hdmi_output_format output_format = vc4_state->output_format;
 	unsigned long long pixel_rate = mode->clock * 1000;
 	unsigned long long tmds_rate;
 	int ret;
@@ -1832,10 +1834,16 @@ static int vc4_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 		pixel_rate = mode->clock * 1000;
 	}
 
+
 	ret = vc4_hdmi_encoder_compute_config(vc4_hdmi, vc4_state, mode);
 	if (ret)
 		return ret;
 
+	/* vc4_hdmi_encoder_compute_config may have changed output_bpc and/or output_format */
+	if (vc4_state->output_bpc != output_bpc ||
+	    vc4_state->output_format != output_format)
+		crtc_state->mode_changed = true;
+
 	return 0;
 }