drm/i915/guc: Move lrc desc setup to where it is needed

The LRC descriptor was being initialised early on in the context
registration sequence. It could then be determined that the actual
registration needs to be delayed and the descriptor would be wiped
out. This is inefficient, so move the setup to later in the process
after the point of no return.

v2: Move some split changes into the split patch (and do them
correctly).

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/20220302003357.4188363-6-John.C.Harrison@Intel.com