drm/etnaviv: Optimize error handling in etnaviv_gem_new_userptr()

Refactor this function implementation so that the
drm_gem_object_unreference_unlocked() function will only be called once
in case of a failure according to the Linux coding style recommendation
for centralized exiting of functions.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
[seanpaul tweaked subject]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/4af34ce6-62c6-7966-1ae3-0877d5ac909d@users.sourceforge.net