drm/msm/dsi-staging: fix the DMS mode flag setting

DFPS validation doesn't update DSI_MODE_FLAG_DMS when there is
only refresh rate change but not resolution change. This is not
expected. DMS should be able to support this use case if DFPS
is not enabled.

Change-Id: I738bce68b1dc098338281ac95156a483769608c4
Signed-off-by: Tharun Raj Soma <tsoma@codeaurora.org>