drm/amd/display: Not doing optimize bandwidth if flip pending.

[Why]
In some scenario like 1366x768 VSR enabled connected with a 4K monitor
and playing 4K video in clone mode, underflow will be observed due to
decrease dppclk when previouse surface scan isn't finished

[How]
In this use case, surface flip is switching between 4K and 1366x768,
1366x768 needs smaller dppclk, and when decrease the clk and previous
surface scan is for 4K and scan isn't done, underflow will happen.  Not
doing optimize bandwidth in case of flip pending.

Signed-off-by: Yongqiang Sun <yongqiang.sun@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>