drm_hwcomposer: Fix SetDisplayBrightness failure
Display brightness is not supported in drm-hwc. VTS expects an error to be returned if the client sets a value for display brightness when it is not supported.
Change-Id: I0a9f8a4fe8aabff91bfb262057cf116d634803ef Signed-off-by: Drew Davenport ddavenport@google.com