drm/i915/uc: Move xfer rsa logic to common function

The way we copy the RSA is the same for GuC and HuC, so we can move the
logic in a common function. this will also make any update needed for
local memory easier.

v2: return the number of copied bytes and check it (Chris)

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190725001813.4740-6-daniele.ceraolospurio@intel.com