drm: bridge: adv7511: Move CEC definitions to adv7511_cec.c

ADV7511_REG_CEC_RX_FRAME_HDR[] and ADV7511_REG_CEC_RX_FRAME_LEN[]
are only used inside adv7511_cec.c.

Move their definitions to this file to avoid the following build
warnings when CONFIG_DRM_I2C_ADV7511_CEC is not selected:

drivers/gpu/drm/bridge/adv7511/adv7511.h:229:17: warning: 'ADV7511_REG_CEC_RX_FRAME_HDR' defined but not used [-Wunused-const-variable=]
drivers/gpu/drm/bridge/adv7511/adv7511.h:235:17: warning: 'ADV7511_REG_CEC_RX_FRAME_LEN' defined but not used [-Wunused-const-variable=]

Reported-by: kernel test robot <lkp@intel.com>
Fixes: ab0af093 ("drm: bridge: adv7511: use non-legacy mode for CEC RX")
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220525215316.1133057-1-festevam@gmail.com