drm/msm/dp: fix mode validation logic

Fix the mode validation logic by assigning a refresh value if the
mode being validated has a refresh value of zero. Furthermore,
extend the mode validation to validate the aspect ratio of the
mode if the debug feature is being used.

CRs-Fixed: 2099403
Change-Id: Icdc61125dccc88478f5c2964d74dd5efa0cde764
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>