drm/i915/guc: Keep the ctx_pool_vaddr mapped, for easy access

The GuC descriptor is big in size. If we use a local definition of
guc_desc we have a chance to overflow stack, so avoid it.

Also, Chris abhors scatterlists :)

v2: Rebased, helper function to retrieve the context descriptor,
s/ctx_pool_vma/ctx_pool/

v3: Zero out guc_context_desc before initialization

v4: Do not do arithmetic on void pointers (Daniele)

v5: Nicer than arithmetic on pointers (Chris, Joonas)

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>