drm/radeon: deal with errors from framebuffer init path.

We've been getting occasional oops running a 32-bit kernel on a certain
system in our RHEL test hw. It appears that we fail to get sufficent ioremap
space for the framebuffer, and this leads to an oops.

This patch should fix the oops and leave a message in the logs we can
check for.

A future fix would probably to resize the console to a size that we can
ioremap.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>