drm/amd/display: Add sysfs interface for set/get srm

drm/amd/display: Add sysfs interface for set/get srm

[Why]
PSP doesn't have the ability to store SRM in a non-volatile memory.  And since
the kernel cannot write to the storage directly, we need usermode to facilitate
this

As per spec the SRM needs to be persistent so this interface is to be
called by the usermode anytime the system goes down/powers on

*boot/resume: load from storage
*shutdown/suspend: save to storage

[How]
Provide a sysfs interface so that the usermode can set/get srm at the right times

save to storage: call "cat /sys/class/drm/card0/device/hdcp_srm > file" after boot and resume
	-driver calls psp_get_srm() to get the stored srm and outputs it

load from storage: call "cat file > /sys/class/drm/card0/device/hdcp_srm" before shutdown and suspend
	-driver reads the file from sysfs and calls psp_set_srm() to send the SRM to PSP

v2:
-update commit description
-add comment about sysfs file handling in the code

v3:
- squash in use after free fix (Dan Carpenter)

Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>