drm/amd/display: Refactor dccg35_get_other_enabled_symclk_fe

[Why]

Function 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]

Refactor the function to correctly return the number of FEs connected
to the input BE. Also, rename it for clarity.

Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Nicholas Susanto <Nicholas.Susanto@amd.com>
Signed-off-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>