drm/vc4: hdmi: Silence pixel clock error on -EPROBE_DEFER

If the vc4 hdmi driver loads before the pixel clock is available we
see a spurious "*ERROR* Failed to get pixel clock" error.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200525012859.267433-1-james.hilliard1@gmail.com