drm/i915/i915_reg.h: fix the checkpatch MACRO_ARG_PRECEDENCE issues

While I don't see any issue with the way these macros are being called
today, let's protect them against operator precedence issues before
they happen.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180612235654.7914-4-paulo.r.zanoni@intel.com