drm/radeon/kms: clean up gart dummy page handling

As per Konrad's original patch, the dummy page used
by the gart code and allocated in radeon_gart_init()
was not freed properly in radeon_gart_fini().

At the same time r6xx and newer allocated and freed the
dummy page on their own.  So to do Konrad's patch one
better, just remove the allocation and freeing of the
dummy page in the r6xx, 7xx, evergreen, and ni code and
allocate and free in the gart_init/fini() functions for
all asics.

Cc: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>