drm/i915/guc: add a helper to allocate and map guc vma

We already have a couple of use-cases in the code and another one will
come in one of the later patches in the series.

v2: use the new function for the CT object as well

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> #v1
Reviewed-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205220243.27403-2-daniele.ceraolospurio@intel.com