drm/vc4: hdmi: Move initial register read after pm_runtime_get

Commit ecdd08fd ("drm/vc4: hdmi: Make sure the device is powered
with CEC") made sure that the device is powered while there is
CEC-related accesses but missed one register read in the variable
declaration.

Move the variable assignment after the pm_runtime_resume_and_get.

Fixes: ecdd08fd ("drm/vc4: hdmi: Make sure the device is powered with CEC")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>