drm/amdgpu: Fix kdoc entry in 'amdgpu_vm_cpu_prepare'

This commit updates described non-existent parameters 'resv' and
'sync_mode', and failed to describe the existing 'sync' parameter.

Fixes the below with gcc W=1:
drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Function parameter or struct member 'sync' not described in 'amdgpu_vm_cpu_prepare'
drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Excess function parameter 'resv' description in 'amdgpu_vm_cpu_prepare'
drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c:50: warning: Excess function parameter 'sync_mode' description in 'amdgpu_vm_cpu_prepare'

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>