When the user query the dram usage of a context, show it the dram usage of
its context, not the user context that is currently running on the device.

This has no effect right now as we only have a single process and a single
context, but this makes the code more ready for multiple process support.
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>