i965: Update physical width/height munging for 2x IMS MSAA.

i965: Update physical width/height munging for 2x IMS MSAA.

I can't find any documentation to explain what ought to be done here, so
I simply guessed based on the pattern I observed in the 4x/8x cases.
It appears to work, but it could be totally wrong.

I was able to find the Sandybridge PRM quote from the comments in the
latest documentation: Shared Functions > 3D Sampler > Multisampled
Surface Behavior.  However, it only mentions 4x MSAA - not even 8x.

After a substantial amount more digging, I was able to find a second
page (incorrectly tagged) which confirmed the formulas in our code for
8x MSAA.  However, that page didn't mention 2x MSAA at all.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>