msm: mdss: cleanup multiple vsync handlers logic

Simplify multiple vsync handlers logic by removing ref counters on
handler structure and manage the same with the ref counter for irq
enable/disable. At the same time ensure that the vsync lock is not being
held while calling vsync enable/disable to avoid potential deadlock.

Change-Id: Id79c67f21a5b2d224b2e6b1d3e0b6aa553c44945
CRs-Fixed: 482320
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>