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 <[email protected]>
Cc: John Harrison <[email protected]>
Cc: Matthew Brost <[email protected]>
Cc: Daniele Ceraolo Spurio <[email protected]>
Signed-off-by: Lucas De Marchi <[email protected]>
Reviewed-by: Daniele Ceraolo Spurio <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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