drm/amdgpu: fix broken debug sdma vram access function

Debug VRAM access through SDMA has several broken parts resulting in
silent MMIO fallback.

BO kernel creation takes the location of the cpu addr pointer, not
the pointer itself for address kmap.

drm_dev_enter return true on success so change access check.

The source BO is reserved but not pinned so find the address using the
cursor offset relative to its memory domain start.

Signed-off-by: Jonathan Kim <jonathan.kim@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>