drm/vkms: Add connectors helpers

drm/vkms: Add connectors helpers

This patch adds the struct drm_connector_helper_funcs with some
necessary hooks. Additionally, it also adds some missing hooks at
drm_connector_funcs.

Changes since V1:
- None
Change since V2:
 Daniel Vetter:
 - Remove vkms_conn_mode_valid

Signed-off-by: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/c8ee28b889234e866ef18bce4216385661c48041.1531359228.git.rodrigosiqueiramelo@gmail.com