drm: Add a plane->reset hook

In general having this can't hurt, and the atomic helpers will need
it to be able to reset the state objects properly. The overall idea
is to reset in the order pixels flow, so planes -> crtcs ->
encoders -> connectors.

v2: Squash in fixup from Ville to correctly deference struct drm_plane
instead of drm_crtc when walking the plane list. Fixes an oops in
driver init and resume.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>