drm/msm/sde: fix check for frame event overflow

Fix check to detect an empty free event list needed when there
are too many in-flight frame events in the sde_crtc.

CRs-Fixed: 2005348
Change-Id: I12c9bf7d449d6582584505a7887b6bdf82f2f0d5
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>