drm/amdgpu: just disallow reading untouched registers

Not sure what the original intention was here, but returning a random piece of
kernel memory to userspace because we didn't set the value at all is clearly
not a good idea.

This patch disallows reading the register and returns
a proper error code instead.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Tom St Denis <tom.stdenis@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>