drm/i915: Cleanup error paths through eb_lookup_vma()

Following the simplification to a single lookup loop in commit
170fa29b ("drm/i915: Simplify eb_lookup_vmas()") and commit
d1b48c1e

 ("drm/i915: Replace execbuf vma ht with an idr"), we can go
one step further and reorder the error paths so that the state of the
local variable obj is always known to the compiler and doesn't need the
uninitialized_var markup to squelch a compiler warning.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170912150752.20411-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>