drm/msm/sde: add connector property to expose custom mode info

SDE DRM doesn't expose custom mode info of a display mode by default.
It populates the information only after the client selects the mode
for the display. This forces the client to invoke a dummy atomic commit
just to set the mode and re-read the connector properties to know the
custom mode info. This change avoids the dummy commit by exposing the
custom mode info of all the display modes through a separate blob
property.

Change-Id: I08b5c5f14b7134364827a55f1b41fbfd7b92a980
Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>