drm: edid: add vrefresh info for 640x480@72Hz mode

Add vertical refresh rate info for 640x480@72Hz mode. This mode
is defined in EST and DMT whose vrefresh is 72Hz. This refresh
rate is calculated as 73Hz by DRM driver because DRM rounds up
the result. So specify this rate in the mode definition and fix
the mismatch.

CRs-Fixed: 2294322
Change-Id: Icb2e0b0524cb55ee1f81cac78101097603c1bb2f
Signed-off-by: Ray Zhang <rayz@codeaurora.org>