drm/amdgpu/debugfs: fix memory leak when amdgpu_virt_enable_access_debugfs failed

Fix memory leak in amdgpu_debugfs_gpr_read not freeing data when
amdgpu_virt_enable_access_debugfs failed.

Fixes: 95a2f917 ("drm/amdgpu: restrict debugfs register access under SR-IOV")
Signed-off-by: Chen Tao <chentao107@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>