drm/amd/display: Ignore phantom DTBCLK requirement

[Description]
Phantom DTBCLK can be calculated different from main because phantom
has no DSC and thus will have a different output BPP. Ignore phantom
DTBCLK requirement and only consider non-phantom DTBCLK requirements.

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>