drm/amd/display: Add null check for phantom pipes in prepare mcache

drm/amd/display: Add null check for phantom pipes in prepare mcache

[WHY&HOW]
Sometimes this function is called with a partially deconstructed phantom
stream toplolgy, and should ignore phantoms with no plane state.

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