Improve ScreenInit & CloseScreen

In ARMSOCScreenInit and CloseScreen retain drm mastership while
creating/destroying scanout buffer.
Failures destroying scanout buffer produce errors instead of asserts.
Clarify usage of CloseScreen & FreeScreen.
Exa CloseScreen now frees exa driver pointer.

Change-Id: Icbce4f311e99d0af7702310fd45a064bb5d8b320
3 files changed