drm/amd/display: Know what a pageflip is

[Why]
We were assuming that any commit with allow_modeset == false
was a pageflip.  This was against drm intention and only
worked by sheer luck

[How]
A pageflip is the change from one framebuffer to another

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>