drm/i915/uc: Plumb the gt through fw_upload

The gt is our new central structure for uc-related code, so we can use
that instead of jumping back to i915 via the fw object. Since we have it
in the upload function it is easy to pass it through the lower levels of
the xfer process instead of continuosly jumping via uc_fw->uc->gt, which
will also make things a bit cleaner for the next patch.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725001813.4740-8-daniele.ceraolospurio@intel.com