drm/vc4: Allocate the right amount of space for boot-time CRTC state.

Without this, the first modeset would dereference past the allocation
when trying to free the mm node.

Signed-off-by: Eric Anholt <eric@anholt.net>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170328201343.4884-1-eric@anholt.net
Fixes: d8dbf44f

 ("drm/vc4: Make the CRTCs cooperate on allocating display lists.")
Cc: <stable@vger.kernel.org> # v4.6+
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>