drm_hwcomposer: Store active mode instead of config index

drm_hwcomposer: Store active mode instead of config index

Since the config indices can change out from under us, store the
actual mode instead of an index. This avoids having to
reset active_config when we get a new list from the driver.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Change-Id: I09a1ffbdde16c44ccc8a1d2865f09a5da22c2e26