drm/i915: protect macro parameters in SWING_SEL_{UPP,LO}WER

Protect the macro parameters with parens in order to avoid priority
issues on macro evaluation when the macro argument is not a single
operand.

This is not a problem today, but it could be in the future. I found
this while reviewing a patch that introduces new callers for the
macros.

v2: Rebase.

Reference: commit 04416108 ("drm/i915/cnl: Add registers related to voltage swing sequences.")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
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/20180323195853.4599-1-paulo.r.zanoni@intel.com