msm: mdss: send frame events on polling mode

Due to potential heavy load in the system, in some cases the system may
fall into polling mode for vsyncs if interrupts are not happening fast
enough. In such cases frame events should still be sent so that sync
fences are properly released.

Change-Id: I41903540921e9b4e9c79de3ad0b6c11c20b30520
CRs-Fixed: 569334
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>