drm/msm/sde: handle prepare_fb later during sec/non-sec transitions

The transition from secure to non-secure happens at the beginning
of the commit sequence in kernel context, but the prepare_fb happens
much before in the user context. Defer the prepare framebuffer to be
handled at a later point in the commit when transitioning from secure
to non-secure usecases, as the domains are attached back only during
the commit sequence.

Change-Id: I2f774dc31f84a52e92e666f07f31dbc44298c7f7
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>