drm/amd/display: Remove unnecessary warning

[why]
The warning message floods the dmesg log on Tonga even
though it is expected to have a pix_clk set to zero,
when the pipe is not active.
[how]
remove the assert

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>