gralloc4-vts: fix GetLargeReservedRegion test

The existing test never assigns a handle to bufferHandle so
mGralloc->getReservedRegion() will always be called with
nullptr and will always return with BAD_BUFFER.

Bug: b/146515640
Test: VtsHalGraphicsMapperV4_0Target
Change-Id: I85dac94956c6b9db5b4a19faeb16fc947f7457e1