drm/amdgpu: Resize BAR0 to the maximum available size, even if it doesn't cover VRAM

This allows BAR0 resizing to be done for cards which don't advertise
support for a size large enough to cover the VRAM but which do
advertise at least one size larger than the default. For example,
my RX 5600 XT, which advertises 256MB, 512MB and 1GB.

Signed-off-by: Darren Salt <devspam@moreofthesa.me.uk>
Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.das@amd.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20210107175017.15893-4-nirmoy.das@amd.com