drm/amd/amdgpu: Fix missing error return on kzalloc failure

Currently the kzalloc failure check just sets reports the failure
and sets the variable ret to -ENOMEM, which is not checked later
for this specific error. Fix this by just returning -ENOMEM rather
than setting ret.

Fixes: 4fb93071 ("drm/amd/amdgpu: remove redundant host to psp cmd buf allocations")
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 1ee9d1a0)