drm/i915: Propagate PCI read/write errors during vga_set_state()

This has very little effect other than log the errors in case of failure,
and we then hope for the best.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>