drm/via: reduce no need mutex_lock area

In function via_mem_alloc`s error branch, DRM_ERROR is protected
in the mutex_lock(&dev->struct_mutex) area.
>From the code, we see that DRM_ERROR is just an error log print
without any struct element, there is no need to protect this.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200814083021.8962-1-bernard@vivo.com