msm: kgsl: Hold pm_qos vote for active rendering

Differentiate between the screen off case where kgsl shouldn't
hold a pm_qos vote and the screen on case where power rails
may be turned off after a long idle.  restore_slumber now
corresponds to the screen on/off while strtstp_sleepwake covers
the power rail collapse.

CRs-Fixed: 386956
Change-Id: Id4318f1aa87d6ad0653fe5db18c5d33bda1cc8bb
Signed-off-by: Lucille Sylvester <lsylvest@codeaurora.org>