drm/vc4: hdmi: Switch to detect_ctx

We'll need the locking context in future patch, so let's convert .detect
to .detect_ctx.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://lore.kernel.org/r/20220829134731.213478-6-maxime@cerno.tech