drm/vc4: Avoid using vrefresh==0 mode in DSI htotal math.

The incoming mode might have a missing vrefresh field if it came from
drmModeSetCrtc(), which the kernel is supposed to calculate using
drm_mode_vrefresh().  We could either use that or the adjusted_mode's
original vrefresh value.

However, we can maintain a more exact vrefresh value (not just the
integer approximation), by scaling by the ratio of our clocks.

v2: Use math suggested by Andrzej Hajda instead.
v3: Simplify math now that adjusted_mode->clock isn't padded.
v4: Drop some parens.

Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20170815234722.20700-2-eric@anholt.net
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>

drivers/gpu/drm/vc4/vc4_dsi.c

@@ -866,7 +866,8 @@ static bool vc4_dsi_encoder_mode_fixup(struct drm_encoder *encoder,
 	adjusted_mode->clock = pixel_clock_hz / 1000 + 1;
 
 	/* Given the new pixel clock, adjust HFP to keep vrefresh the same. */
-	adjusted_mode->htotal = pixel_clock_hz / (mode->vrefresh * mode->vtotal);
+	adjusted_mode->htotal = adjusted_mode->clock * mode->htotal /
+				mode->clock;
 	adjusted_mode->hsync_end += adjusted_mode->htotal - mode->htotal;
 	adjusted_mode->hsync_start += adjusted_mode->htotal - mode->htotal;