drm/i915/uc: Prepare for different firmware key sizes

Future GuC/HuC firmwares might be signed with different key sizes.
Don't assume that it must be always 2048 bits long.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211211000756.1698923-3-daniele.ceraolospurio@intel.com