drm/msm/sde: add debugfs to disable/enable uidle

This change adds support to disable/enable uidle
feature through debugfs. Target must have the
uidle feature enabled at boot time to control the
feature runtime using this debugfs.

Change-Id: Icda74d371c13e440d4f1b0e307dcb212b7e2de69
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>