drm/etnaviv: Delete unnecessary checks before two function calls

The functions drm_gem_object_unreference_unlocked() and vunmap() perform
also input parameter validation.
Thus the tests around their calls are not needed.

This issue was detected by using the Coccinelle software.

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/9638cd74-ffc5-d9ee-a40c-9b60e860ad8b@users.sourceforge.net