Fix getDisplayCapabilitiesBasic VTS test

Old test checked error code when calling getBrightnessSupport in the
getDisplayCapabilitiesBasic test.  However, it is possible that the
function is not implemented (UNSUPPORTED).  We want to return normally
and use the returned supported boolean to check this instead.

Bug: 132043561, 132056857
Test: VtsHalGraphicsComposerV2_3TargetTest
Change-Id: I94f8f39ba9129c4a8d939a5385538db3489c46b9