drm/amd/display: Allow pflips from a framebuffer to itself

[Why]
IGT expects that pageflips can be triggered with the same
framebuffer before and after the commit

[How]
Expand the definition of pageflip to include any change
with an old framebuffer and a new framebuffer, even if they're
the same

Signed-off-by: David Francis <David.Francis@amd.com>
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>