PCI: Export pci_rebar_get_possible_sizes()

Export pci_rebar_get_possible_sizes() for use by modular drivers.

Signed-off-by: Darren Salt <devspam@moreofthesa.me.uk>
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20210107175017.15893-2-nirmoy.das@amd.com