drm/edid: Fix EDID Established Timings I and II

Three of the VESA DMT timings in edid_est_modes[] are slightly off.
1. 640x480@72Hz vsync_end should be 492, not 491.
2. 640x480@60Hz clock should be 25175, not 25200.
3. 1024x768@75Hz clock should be 78750, not 78800.
This patch corrects those timings per the VESA DMT specification, and
thus brings them into line with the identical timings in drm_dmt_modes[].

Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160402100817.B60776E23A@gabe.freedesktop.org