drm/modes: tag unused variables to avoid warnings

Some variables are set but never used. To avoid warning when compiling
with W=1 and keep the algorithm like it is tag theses variables
with _maybe_unused macro.
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210102437.19377-1-benjamin.gaignard@st.com

drivers/gpu/drm/drm_modes.c

@@ -233,7 +233,7 @@ struct drm_display_mode *drm_cvt_mode(struct drm_device *dev, int hdisplay,
 		/* 3) Nominal HSync width (% of line period) - default 8 */
 #define CVT_HSYNC_PERCENTAGE	8
 		unsigned int hblank_percentage;
-		int vsyncandback_porch, vback_porch, hblank;
+		int vsyncandback_porch, __maybe_unused vback_porch, hblank;
 
 		/* estimated the horizontal period */
 		tmp1 = HV_FACTOR * 1000000  -
@@ -386,9 +386,10 @@ drm_gtf_mode_complex(struct drm_device *dev, int hdisplay, int vdisplay,
 	int top_margin, bottom_margin;
 	int interlace;
 	unsigned int hfreq_est;
-	int vsync_plus_bp, vback_porch;
-	unsigned int vtotal_lines, vfieldrate_est, hperiod;
-	unsigned int vfield_rate, vframe_rate;
+	int vsync_plus_bp, __maybe_unused vback_porch;
+	unsigned int vtotal_lines, __maybe_unused vfieldrate_est;
+	unsigned int __maybe_unused hperiod;
+	unsigned int vfield_rate, __maybe_unused vframe_rate;
 	int left_margin, right_margin;
 	unsigned int total_active_pixels, ideal_duty_cycle;
 	unsigned int hblank, total_pixels, pixel_freq;