drm/msm/sde: update secure_level after sec to non-sec transition

The sde_kms maintains the global smmu state which holds the
current security level maintained. Set this before a
non-sec to sec transition and unset it after completing the
sec to non-sec transition. This helps to return a valid value
even during transitions. As part of the change add another
intermediate state to check while determining if secure-ui
session is in progress.

Change-Id: I0a5a25c44e6d54923fcc94c98171a7023ddbc0af
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>