drm/msm: update flag for seamless dynamic framerates

Update the definition of the DRM_MODE_FLAG seamless mode changes so
that its value is unique. This change will ensure that there no
potential issues when using it in conjunction with other flags.

Change-Id: I256854631d8a8a2e441a7c6ae7e89f9ff27228c2
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>