drm/radeon: fix radeon kms framebuffer device

smem.start is a physical address which kernel can remap to access
video memory of the fb buffer. We now pin the fb buffer into vram
by doing so we are loosing vram but fbdev need to be reworked to
allow change in framebuffer address.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>