drm/i915: Ignore a hung GPU when flushing the framebuffer prior to a switch

If the gpu is hung, then whatever was inside the render cache is lost
and there is little point waiting for it. Or complaining if we see an
EIO or EAGAIN instead. So, if the GPU is indeed in its death throes when
we need to rewrite the registers for a new framebuffer, just ignore the
error and proceed with the update.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>