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.

Reported-by: Fusheng Huang <fusheng.huang@ecarxgroup.com>
Signed-off-by: Julia Zhang <Julia.Zhang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org