drm/i915: add parentheses around PIXCLK_GATE definitions

By looking at the current way we're using these definitions I don't
think this commit will fix any bug, but programmers from the future
are evil and will certainly find ways to combine macro expansion with
operator precedence to introduce bugs that are hard to find.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>