drm/radeon: Correctly clean up failed display probing

Improve the fbdev probing function to fully clean up if it failed.
Allows to remove special cases from fb_destroy as well.

This change is reorders the operations within radeonfb_probe(). It
first allocates a buffer object, then builds a DRM framebuffer for
the object and finally creates the fbdev device. If every step
succeeded, the probe function clears the framebuffer memory. This
is the optimal order to rollback any changes if any of the steps
fails.

No functional changes.

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>