drm/amdgpu/si: Call amdgpu_ttm_set_active_vram_size from si_dma_start/stop

Without this, we were only ever using the first 256MB of VRAM.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97593


Tested-by: Konstantin A. Lepikhov <lakostis@altlinux.org>
Tested-by: Arek Ruśniak <arek.rusi@gmail.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>