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

[Joe Perches]

On Tue, 2017-10-24 at 17:26 +0300, Dan Carpenter wrote:
> The point of unwind code is to undo what was done earlier.  If a
> function allocates a list of things, using standard unwind style makes
> it simpler, safer and more readable.
> 
> This isn't the case here.  Instead of making the code more readable,
> we're making it more convoluted.  It's just that two out of three error
> messages happened to be the same and Markus wants to save a bit of
> memory by using the same string.  The memory savings is not so big that
> it's worth making the code less readable.

I agree with Dan.

It doesn't save any real memory either as the compiler/linker
reuses the repeated string.

It might, depending on the compiler, save a few bytes of
object code as the compiler may not optimize the repeated
call away though.  But a good compiler could do that too.