drm/hisilicon: Delete the empty function mode_valid

Based on the drm_connector_mode_valid, if the hibmc implementation
of mode_valid only returns MODE_OK, then we need not implement the
mode_valid function.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1610102465-36501-1-git-send-email-tiantao6@hisilicon.com