drm/bridge: it6505: Fix build error

If DRM_ITE_IT6505 is y but DRM_DP_HELPER is m, building failed:

drivers/gpu/drm/bridge/ite-it6505.o: In function `it6505_i2c_remove':
ite-it6505.c:(.text+0x35c): undefined reference to `drm_dp_aux_unregister'
drivers/gpu/drm/bridge/ite-it6505.o: In function `it6505_dpcd_read':
ite-it6505.c:(.text+0x420): undefined reference to `drm_dp_dpcd_read'
drivers/gpu/drm/bridge/ite-it6505.o: In function `it6505_get_dpcd':
ite-it6505.c:(.text+0x4a4): undefined reference to `drm_dp_dpcd_read'
drivers/gpu/drm/bridge/ite-it6505.o: In function `it6505_dpcd_write':
ite-it6505.c:(.text+0x52c): undefined reference to `drm_dp_dpcd_write'

Select DRM_DP_HELPER for DRM_ITE_IT6505 to fix this.

Fixes: b5c84a9e ("drm/bridge: add it6505 driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.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/20220317094724.25972-1-yuehaibing@huawei.com