drm/amd/display: Modify the hdcp device count check condition

[why]
Some MST display may not report the internal panel to DEVICE_COUNT,
that makes the check condition always failed.

[how]
To update this condition with the reported device count + 1
(because the immediate repeater's internal panel is possibly
not included in DEVICE_COUNT)

Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Acked-by: Bindu Ramamurthy <bindu.r@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>