drm/i915/selftests: Pass intel_context to mock_request

Modernise the mock_request factory to take intel_context not a (GEM
context, intel_engine_cs) tuple.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190808115640.20552-1-chris@chris-wilson.co.uk