drm/amdgpu: avoid using null object of framebuffer

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: avoid using null object of framebuffer

Instead of using state->fb->obj[0] directly, get object from framebuffer

by calling drm_gem_fb_get_obj() and return error code when object is

null to avoid using null object of framebuffer.