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 <[e-mail]>
Cc: Alex Deucher <[e-mail]>
Signed-off-by: Srinivasan Shanmugam <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm_cpu.c

···
  * amdgpu_vm_cpu_prepare - prepare page table update with the CPU
  *
  * @p: see amdgpu_vm_update_params definition
- * @resv: reservation object with embedded fence
- * @sync_mode: synchronization mode
+ * @sync: sync obj with fences to wait on
  *
  * Returns:
  * Negativ errno, 0 for success.