drm: fix EDID parser problem with positive/negative hsync/vsync

Comparing the layouts of struct detail_pixel_timing with
x.org's struct detailed_timings and how those are handled,
it appears that the hsync_positive and vsync_positive
fields are backwards.

This patch fixes https://bugs.freedesktop.org/show_bug.cgi?id=20019
for me. It was tested on 2 monitors, LG FLATRON L225WS 22" and
a YAKUMO 17" for which more details are unknown.

Signed-off-by: Pantelis Koukousoulas <pktoss@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>