drm/i915/context: switch contexts with execbuf2

Use the rsvd1 field in execbuf2 to specify the context ID associated
with the workload. This will allow the driver to do the proper context
switch when/if needed.

v2: Add checks for context switches on rings not supporting contexts.
Before the code would silently ignore such requests.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>