drm/amd/display: Initialize attribute for hdcp_srm sysfs file

It is stored in dynamically allocated memory, so sysfs_bin_attr_init() must
be called to initialize it. (Note: "initialization" only sets the .attr.key
member in this struct; it does not change the value of any other members.)

Otherwise, when CONFIG_DEBUG_LOCK_ALLOC=y this message appears during boot:

    BUG: key ffff9248900cd148 has not been registered!

Fixes: 9037246b ("drm/amd/display: Add sysfs interface for set/get srm")
Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1586


Reported-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Signed-off-by: David Ward <david.ward@gatech.edu>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org