drm/i915: Don't leak primary/cursor planes on crtc init failure

drm/i915: Don't leak primary/cursor planes on crtc init failure

Call intel_plane_destroy() instead of drm_plane_cleanup() so that we
also free the plane struct itself when bailing out of the crtc init.

And make intel_plane_destroy() NULL tolerant to avoid having to check
for it in the caller.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464371966-15190-5-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>