drm/amd/display: drop vblank_lock from struct amdgpu_display_manager

As of commit 09a5df6c ("drm/amd/display: Fix multi-display support
for idle opt workqueue"), vblank_lock is no longer being used. So, don't
init it in amdgpu_dm_init() and remove it from struct
amdgpu_display_manager.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>