drm/vc4: hdmi: Add missing clk_disable_unprepare on error path

In vc4_hdmi_encoder_pre_crtc_configure, if clk_request_start for the HSM
clock fails, we don't call clk_disable_unprepare on the pixel clock even
though it's enabled by now.

Make sure it's there to avoid leaking that reference.

Fixes: cd4cb49d ("drm/vc4: hdmi: Adjust HSM clock rate depending on pixel rate")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>