drm/vc4_hdmi: Force modeset when bpc 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 changes property to "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.

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