drm/amdgpu: Add vm context module param

Add VM update mode module param (amdgpu.vm_update_mode) that can used to
control how VM pde/pte are updated for Graphics and Compute.

BIT0 controls Graphics and BIT1 Compute.
 BIT0 [= 0] Graphics updated by SDMA [= 1] by CPU
 BIT1 [= 0] Compute updated by SDMA [= 1] by CPU

By default, only for large BAR system vm_update_mode = 2, indicating
that Graphics VMs will be updated via SDMA and Compute VMs will be
updated via CPU. And for all all other systems (by default)
vm_update_mode = 0

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