drm/i915: Invariably invalidate before ctx switch

We have been setting the bit which was originally BIOS dependent since:
commit f05bb0c7
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sun Jan 20 16:33:32 2013 +0000

    drm/i915: GFX_MODE Flush TLB Invalidate Mode must be '1' for scanline waits

Therefore, we do not need to try to figure it out dynamically and we can
just always invalidate the TLBs.

It's a partial revert of:
commit 12b0286f


Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Mon Jun 4 14:42:50 2012 -0700

    drm/i915: possibly invalidate TLB before context switch

The original commit attempted to only invalidate when necessary
(very much a relic from the old days). Now, we can just always invalidate.

I guess the old TODO still exists. Since we seem to have abandoned ILK
contexts however, there isn't much point in even remembering.

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