drm/i915/bios: cleanup return type of intel_parse_bios()

These are unintuitive.  These are type bool and return -1 casted to true
on failure.  Let's just make it return an int.  The callers don't care,
but let's change this as a cleanup.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>