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 04416108ccea ("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

drivers/gpu/drm/i915/i915_reg.h

@@ -1749,9 +1749,9 @@ enum i915_power_well_id {
 #define ICL_PORT_TX_DW2_LN0(port)	_MMIO_PORT(port, \
 						   _ICL_PORT_TX_DW2_LN0_A, \
 						   _ICL_PORT_TX_DW2_LN0_B)
-#define   SWING_SEL_UPPER(x)		((x >> 3) << 15)
+#define   SWING_SEL_UPPER(x)		(((x) >> 3) << 15)
 #define   SWING_SEL_UPPER_MASK		(1 << 15)
-#define   SWING_SEL_LOWER(x)		((x & 0x7) << 11)
+#define   SWING_SEL_LOWER(x)		(((x) & 0x7) << 11)
 #define   SWING_SEL_LOWER_MASK		(0x7 << 11)
 #define   RCOMP_SCALAR(x)		((x) << 0)
 #define   RCOMP_SCALAR_MASK		(0xFF << 0)