drm/i915: Fix context size calculation on SNB/IVB/VLV

All the different context sizes reported in the CXT_SIZE register
aren't meant to be simply added together.

While BSpec is somewhat unclear on the topic of the actual context
size, empirical tests have now revealed the truth. So let's add a
big fat comment to remind people how it all works.

As a result of correctly interpreting CXT_SIZE, the IVB context
size is reduced from three pages to two, while SNB context size
remains at two pages.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>