drm/msm/dp: use fixed NVID for VGA dongles

Most VGA dongles expect the source to use a fixed NVID value of
0x8000 even though the source may be operating in a synchronous
clocking mode. Using any other values for NVID may result in
no display on the attached VGA sink. Add support for this by
calculating the correct MVID based on the fixed NVID value.

CRs-Fixed: 2092000
Change-Id: Ib7b3e1d6508baea16e068533324ea91a92a5af39
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>