drm/i915: Make sure we respect n.max on VLV

We limit the maximum n divider value in order to make sure the PLL's
reference inout is at least 19.2 MHz. I assume that is done to satisfy
some hardware requirement.

However we never check whether that calculated limit is below the
maximum supoorted N divider value (7). In practice that is always true
since we only support 100 MHz reference clock, but making the code
safe against higher reference clocks seems like a reasoanble thing to
do.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -678,15 +678,16 @@ vlv_find_best_dpll(const intel_limit_t *limit, struct drm_crtc *crtc,
 		   intel_clock_t *best_clock)
 {
 	intel_clock_t clock;
-	u32 minupdate = 19200;
 	unsigned int bestppm = 1000000;
+	/* min update 19.2 MHz */
+	int max_n = min(limit->n.max, refclk / 19200);
 
 	target *= 5; /* fast clock */
 
 	memset(best_clock, 0, sizeof(*best_clock));
 
 	/* based on hardware requirement, prefer smaller n to precision */
-	for (clock.n = limit->n.min; clock.n <= ((refclk) / minupdate); clock.n++) {
+	for (clock.n = limit->n.min; clock.n <= max_n; clock.n++) {
 		for (clock.p1 = limit->p1.max; clock.p1 > limit->p1.min; clock.p1--) {
 			for (clock.p2 = limit->p2.p2_fast; clock.p2 > 0;
 			     clock.p2 -= clock.p2 > 10 ? 2 : 1) {