drivers: gpu: drm: bridge: fix kconfig dependency on DRM_KMS_HELPER

When DRM_TOSHIBA_TC358762 is enabled and DRM_KMS_HELPER is disabled,
Kbuild gives the following warning:

WARNING: unmet direct dependencies detected for DRM_PANEL_BRIDGE
  Depends on [n]: HAS_IOMEM [=y] && DRM_BRIDGE [=y] && DRM_KMS_HELPER [=n]
  Selected by [y]:
  - DRM_TOSHIBA_TC358762 [=y] && HAS_IOMEM [=y] && DRM [=y] && DRM_BRIDGE [=y] && OF [=y]

This is because DRM_TOSHIBA_TC358762 selects DRM_PANEL_BRIDGE,
without depending on or selecting DRM_KMS_HELPER,
despite that config option depending on DRM_KMS_HELPER.

Signed-off-by: Julian Braha <julianbraha@gmail.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210222215502.24487-1-julianbraha@gmail.com