drm/msm: Enable clocks during enable/disable_vblank() callbacks

AHB clock should be enabled before accessing registers during
enable/disable_vblank(). Since these 2 callbacks are called in
atomic context while clk_prepare may cause thread sleep, a work
is scheduled to control vblanks.

v2: fixup spinlock initialization

Signed-off-by: Hai Li <hali@codeaurora.org>
[add comment about cancel_work_sync() before drm_irq_uninstall()]
Signed-off-by: Rob Clark <robdclark@gmail.com>