drm/amd/display: Add null pointer check in mod_hdcp_hdcp1_create_session()

The function mod_hdcp_hdcp1_create_session() calls the function
get_first_active_display(), but does not check its return value.
The return value is a null pointer if the display list is empty.
This will lead to a null pointer dereference.

Add a null pointer check for get_first_active_display() and return
MOD_HDCP_STATUS_DISPLAY_NOT_FOUND if the function return null.

This is similar to the commit c3e9826a
("drm/amd/display: Add null pointer check for get_first_active_display()").

Fixes: 2deade5e ("drm/amd/display: Remove hdcp display state with mst fix")
Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>