drm/amdgpu: Fix extra call to amdgpu_ctx_put.

In amdgpu_cs_parser_init() in case of error handling
amdgpu_ctx_put() is called without setting p->ctx to NULL after that,
later amdgpu_cs_parser_fini() also calls amdgpu_ctx_put() again and
mess up the reference count.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>