drm/msm/dsi-staging: fix dynamic fps mode checks and tracking

Do not store one-time transient mode flags into the DSI panel's
current mode. Do not fail when DSI is asked to transition to the
same fps.

CRs-Fixed: 2005394
Change-Id: I89d6bc8eb8d7fd8cd6aa5ba3f493c4edd348bba5
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>