drm/amd/display: Move drm_get_vblank from legacy code

Previously, we assumed that allow_modeset=false => page flip. This
assumption breaks when an atomic commit is submitted with allow_modeset
set to false, since the legacy flip code is never called (the legacy
code grabs the vblank reference).

Fix: Move drm_vblank_get() from amdgpu_atomic_helper_page_flip() to
amdgpu_dm_commit_surfaces().

Signed-off-by: Leo (Sunpeng) Li <sunpeng.li@amd.com>
Reviewed-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>