KMS: fix EDID detailed timing frame rate

When KMS has parsed an EDID "detailed timing", it leaves the frame rate
zeroed.  Consecutive (debug-) output of that mode thus yields 0 for
vsync.  This simple fix also speeds up future invocations of
drm_mode_vrefresh().

While it is debatable whether this qualifies as a -stable fix I'd apply
it for consistency's sake; drm_helper_probe_single_connector_modes()
does the same thing already for all probed modes.

Cc: stable@vger.kernel.org
Signed-off-by: Torsten Duwe <duwe@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>