drm/i915/selftests: Teach igt_gpu_fill_dw() to take intel_context

Avoid having to pass around (ctx, engine) everywhere by passing the
actual intel_context we intend to use. Today we preach this lesson to
igt_gpu_fill_dw and its callers' callers.

The immediate benefit for the GEM selftests is that we aim to use the
GEM context as the control, the source of the engines on which to test
the GEM context.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823235141.31799-1-chris@chris-wilson.co.uk