msm: mdss: replace uevent based hw vsync events with sysfs poll

Replace hw vsync ctrl logic with ability to read vsync timestamps
through sysfs file when vsync ctrl is enabled from userspace.

CRs-Fixed: 407684
Change-Id: I0fb973592090ed9ee81bc3d54f53a18f4c131e0f
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>