drm/radeon: Remove test for !screen_base in fbdev probing

The screen_base field comes from the fbdev BO and contains the fbdev
framebuffer base address. We get the BO memory via radeon_bo_kmap(),
which already reports the error -ENOMEM if the buffer could not be
mapped. So remove the later test for screen_base, which will never
be NULL at this point.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>