drm: increase maximum drm open count to 128

Currently maximum drm open count is set to 20, which is not enough
for AGL platform. Increase the number to 128 to meet AGL requirement.

Change-Id: Idf01c10615c65a80d96a088c0d92c59b354c0f5f
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>