drm/amd/display: Refactor dccg401_get_other_enable_symclk_fe

drm/amd/display: Refactor dccg401_get_other_enable_symclk_fe

[WHY]
Function was used to check the number of FEs connected to the current BE.
This was then used to determine if the symclk could be disabled, if all
FEs were disconnected. However, the function would skip over the primary
FE and return 0 when the primary FE was still connected. This caused black
screens on driver disable with an MST daisy chain hooked up.

[HOW]
Refactored the function to correctly return the number of FEs connected to
the input BE. Also renamed it for clarity.

Reviewed-by: Wenjing Liu <wenjing.liu@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Relja Vojvodic <relja.vojvodic@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>