disp: msm: sde: destroy framebuffers after plane cleanup

Add framebuffer destroy immediately following plane cleanup
during secure display transition. Set state's fb to NULL
after destroying to avoid attempting to destroy fb again
during plane state destroy. Previously, framebuffer destroy
is happening after the context is detached while plane states
are being destroyed.

Change-Id: I273ce5b85c30962ea7e0a738a366487c9c85d4df
Signed-off-by: Samantha Tran <samtran@codeaurora.org>