drm/amd/display: Clean up style problems in amdgpu_dm_hdcp.c

Conform to Linux kernel coding style.

And promote sysfs entry for set/get srm to kdoc.
Suggested-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>