drm/atomic: Create __drm_atomic_helper_crtc_reset() for subclassing crtc_state.

We already have __drm_atomic_helper_connector_reset() and
__drm_atomic_helper_plane_reset(), extend this to crtc as well.

This will allow us to set default values in the crtc_state, without
having to do it in each driver separately.

Of all drivers that need conversion, only nouveau is done in this
commit, because it wrote its own __drm_atomic_helper_crtc_reset(),
clashing with the drm core.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190301125627.7285-3-maarten.lankhorst@linux.intel.com