drm/amd/display: determine rx id list bytes to read based on device count

[why]
Some RX doesn't like us to read rx id list upto max rx id list size.  As
discussed, we decided to read rx id list based on device count.

[how]
According to HDCP specs the actual size of rx id list is calculated as
rx id list size = 2+3+16+5*device_count.  We will read 16 bytes at a
time until it reached or exceeded rx id list size.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>