msm: mdss: add support for LP2 low power state

Define a new low power state for display (LP2), in which the panel is
on but cannot accept any updates. Add support for transitions in and
out of this new power state.

Change-Id: Ie25ec157bf6bb16efb76d535822c2e978a0bc2e1
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>
Signed-off-by: Aarushi Girdhar <agirdhar@codeaurora.org>