drm/amdgpu: cleanup coding style in amdkfd a bit

Make the code a bit more readable by using a common
error handling pattern.

Signed-off-by: Bernard Zhao <bernard@vivo.com>
Reviewed-by: Christian König <christian.koenig@amd.com&gt;.>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>