drm/amdgpu/display: clean up hdcp workqueue handling

Use the existence of the workqueue itself to determine when to
enable HDCP features rather than sprinkling asic checks all over
the code.  Also add a check for the existence of the hdcp
workqueue in the irq handling on the off chance we get and HPD
RX interrupt with the CP bit set.  This avoids a crash if
the driver doesn't support HDCP for a particular asic.

Fixes: 96a3b32e ("drm/amd/display: only enable HDCP for DCN+")
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=206519


Reviewed-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>