drm/msm/dsi-staging: fix memory leak issue in get modes call

There is a potential memory leak if display get modes gets
called multiple times. This change avoids allocating new modes
if any previously probed modes are present.

Change-Id: Ia027d93204605b80f071609c896ccdf9a87d52ce
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>