drm/ast: Fixed 1280x800 Display Issue

drm/ast: Fixed 1280x800 Display Issue

BugLink: http://bugs.launchpad.net/bugs/1786352
[ Upstream commit 5a9f698feb11b198f17b2acebbfe0e2716a3beed ]

The original ast driver cannot display properly if the resolution is 1280x800 and the pixel clock is 83.5MHz.
Here is the update to fix it.

Signed-off-by: Y.C. Chen <yc_chen@aspeedtech.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>