drm/i915/guc: Prepare for error propagation

Currently guc_mmio_reg_add() relies on having enough memory available in
the array to add a new slot. It uses
`GEM_BUG_ON(count >= regset->size);` to protect going above the
threshold.

In order to allow guc_mmio_reg_add() to handle the memory allocation by
itself, it must return an error in case of failures.  Adjust return code
so this error can be propagated to the callers of guc_mmio_reg_add() and
guc_mmio_regset_init().

No intended change in behavior.

Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220208070141.2095177-2-lucas.demarchi@intel.com