disp: msm: sde: avoid drm_crtc_vblank_on during seamless transition

When there is a race condition between DMS seamless transition and
drm_wait_vblank_ioctl, the latter gets deregistered for vblank
handle as drm_crtc_vblank_on call in crtc enbale increments vblank
count. This change avoids drm_crtc_vblank_on call during seamless
transition when crtc is already enabled as it is not required.

Change-Id: I0b9327a98cef00405b5b94e24a3fd15205339cfc
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>