drm/amdgpu: limit visible vram if it's smaller than the BAR

In some cases the amount of vram may be less than the BAR size,
if so, limit visible vram to the amount of actual vram, not the
BAR size.

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