drm/amd/display: Remove inaccessible registers from DMU diagnostics

[Why]
SEC_CNTL isn't readable by x86 and can block Z8 entry if read.

[How]
Remove the read.

Reviewed-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>