drm/i915/gvt: Use common error handling code in shadow_workload_ring_buffer()

[SF Markus Elfring]

>> Add a jump target so that a call of the function "gvt_vgpu_err" is stored
>> only once at the end of this function implementation.
>> Replace two calls by goto statements.
>>
>> This issue was detected by using the Coccinelle software.
> 
> I don't think this is an issue or an improvement.

Do you care for the detail how often an error message is stored in the code?

Regards,
Markus