drm/amdgpu: Reserved vram for smu to save debug info.

v2: check reserved vram size before allocate.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>