drm: Add __drm_atomic_helper_crtc_state_reset() & co.

drm: Add __drm_atomic_helper_crtc_state_reset() & co.

Annoyingly __drm_atomic_helper_crtc_reset() does two
totally separate things:
a) reset the state to defaults values
b) assign the crtc->state pointer

I just want a) without the b) so let's split out part
a) into __drm_atomic_helper_crtc_state_reset(). And
of course we'll do the same thing for planes and connectors.

v2: Fix conn__state vs. conn_state typo (Lucas)
    Make code and kerneldoc match for
    __drm_atomic_helper_plane_state_reset()

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107142417.11107-1-ville.syrjala@linux.intel.com
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>