drm/i915/bxt: Fix wrongly placed ')' in I915_READ()

Not the first time! not the last time?

There is a possibility to use gcc 5's -Wbool-compare to try and compare
(reg) in those macros to a constant and gcc will warn that the
comparison between a boolean expression and a constant is always either
true or false. Maybe.

Cc: Imre Deak <imre.deak@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>