drm/amdgpu: use IS_ERR for debugfs APIs

debugfs APIs returns encoded error so use
IS_ERR for checking return value.

v2: return PTR_ERR(ent)

References: drm/amd#1686


Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-By: Shashank Sharma <shashank.sharma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>