drm/msm/sde: switch to lp2 for pm suspend call during lp1

If a pm suspend event is received while the display is in
LP1 power saving mode, switch to LP2 and leave the display
enabled rather than powering off normally. This is to satisfy
"always on" use cases that require the display panel to
be enabled while the rest of the system is suspended.

Change-Id: I2091f9baa159891a612029e01b829c32b0c13c9e
Signed-off-by: Clarence Ip <cip@codeaurora.org>