drm/armada: Remove NULL open/pre/postclose hooks

The compiler will do this, but the void hits when grepping all the
hooks for a subsystem wide audit are slightly annoying. So remove them
for next time around.

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452548477-15905-8-git-send-email-daniel.vetter@ffwll.ch


Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>