drm/amd/display: handle invalid connector indices

[Why]
The function to count the number of valid connectors does not
guarantee that the first n indices are valid, only that there
exist n valid indices. When invalid indices are present, this
results in later valid connectors being missed, as processing
would end after checking n indices.

[How]
 - count valid indices separately from total indices examined
 - add explicit definition of MAX_LINKS

Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>