drm/amd/display: Guard against zero memory channels

[Why]
If BIOS doesn't specify number of memory channels then bandwidth
validation will fail due to insufficient BW in DML.

[How]
If BIOS is setting zero channels then use the default in the table.
If no entry is in the table and no BIOS value is specified then
throw an ASSERT for future developers to look into.

Reviewed-by: Michael Strauss <Michael.Strauss@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>