drm/amd/amdgpu/amdgpu_vm_cpu: Fix 'amdgpu_vm_cpu_prepare()'s doc-rot

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Function parameter or member 'resv' not described in 'amdgpu_vm_cpu_prepare'
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Function parameter or member 'sync_mode' not described in 'amdgpu_vm_cpu_prepare'
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Excess function parameter 'owner' description in 'amdgpu_vm_cpu_prepare'
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Excess function parameter 'exclusive' description in 'amdgpu_vm_cpu_prepare'

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>