drm/i915: Initialise GTT MTRR to -1

Fixes a regression from 9e984bc1 (drm/i915: Don't do MTRR setup if PAT
is enabled) where we left the MTRR as 0 and so tried to free a MTRR we
did not own during unload.

Reported-and-tested-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>