fbdev: Fix fb_find_nearest_mode refresh comparison

Refresh rate nearness is not calculated or reset when nearest resolution
changes.

This patch resets the refresh rate differential measurement whenever a
new nearest resolution is discovered.  This fixes two error cases;
first, wherein the first mode's refresh rate differential is never
calculated and second, when the closest refresh rate from a previous
nearest resolution is erroneously preserved.

Signed-off-by: Andrew Kephart <andrew.kephart@alereon.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>