drm/vgem: Simplify dumb_map

The offset manager already checks for existing offsets internally,
while holding suitable locks. We can drop this check.

v2: Fix title (Emil).

Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-11-git-send-email-daniel.vetter@ffwll.ch