drm/i915/uc: Remove redundant RSA offset definition

According to Firmware layout definition, RSA signature is located
after CSS header and uCode so actual RSA offset in the blob can be
easily calculated when needed (and we need it only once).

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190726184212.1836-3-michal.wajdeczko@intel.com