drm/radeon: implement simple doorbell page allocator

The doorbell aperture is a PCI BAR whose pages can be
mapped to compute resources for things like wptrs
for userspace queues.

This patch maps the BAR and sets up a simple allocator
to allocate pages from the BAR.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/cik.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon.h
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_device.c
Simple merge