drm/bridge/sii8620: fix dependency on extcon

The DRM_SIL_SII8620 kconfig has a weak `imply` dependency
on EXTCON, which causes issues when sii8620 is built
as a builtin and EXTCON is built as a module.

The symptoms are 'undefined reference' errors caused
by the symbols in EXTCON not being available
to the sii8620 driver.

Fixes: 68883844 ("drm/bridge/sii8620: use micro-USB cable detection logic to detect MHL")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419090124.153560-1-robert.foss@linaro.org