drm/i915: Clarify relocation errnos

While trying to find a random -EINVAL from a failing test, I noticed we
had a few hard to follow return values.

The first two hunks in this patch replace completely useless
initialization of ret. The last several hunks help to distinguish
between altering 'return ret' and 'return <ERROR>'

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>