drm/amd/amdgpu: Add write() method to VRAM debugfs entry (v2)

Allows writing data to vram via debugfs.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>

(v2):  Call get_user before holding spinlock.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>