drm/i915: Fix PSR programming

| has a higher precedence than ?. Therefore, the calculation doesn't do
at all what you would expect. Thanks to Ken for convincing me that this
was indeed the issue. Send me back to C programmer school, please.

I'm sort of surprised PSR was continuing to work for people. It should
be broken IMO (and it was broken for me, but I had assumed it never
worked).

Regression from:
commit ed8546ac


Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Mon Nov 4 22:45:05 2013 -0800

    drm/i915/bdw: Support eDP PSR

Cc: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Cc: Kenneth Graunke <kenneth.w.graunke@intel.com>
Cc: Art Runyan <arthur.j.runyan@intel.com>
Reported-by: "Kumar, Kiran S" <kiran.s.kumar@intel.com>
Cc: stable@vger.kernel.org [v3.13+]
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>