drm/i915/guc: Fix potential invalid pointer dereferences when decoding G2Hs

Some G2H handlers were reading the context id field from the payload
before checking the payload met the minimum length required.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220302003357.4188363-9-John.C.Harrison@Intel.com