drm/i915: Dont clear PIN_GLOBAL in the execbuf pinning fallback

PIN_GLOBAL is set only when userspace asked for it, and that
is only the case for the gen6 PIPE_CONTROL workaround. We're not
allowed to just clear this.

The important part of the fallback is to drop the restriction to
the mappable range.

This issue has been introduced in

commit edf4427b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Jan 14 11:20:56 2015 +0000

    drm/i915: Fallback to using CPU relocations for large batch buffers

v2: Chris pointed out that we also miss to set PIN_GLOBAL when the
buffer is already bound. Fix this up too.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>