drm/radeon: Fix vram_size/visible values in DRM_RADEON_GEM_INFO ioctl

vram_size is supposed to be the total amount of VRAM that can be used by
userspace, which corresponds to the TTM VRAM manager size (which is
normally the full amount of VRAM, but can be just the visible VRAM when
DMA can't be used for BO migration for some reason).

The above was incorrectly used for vram_visible before, resulting in
generally too large values being reported.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>