drm/amd/display: avoid using null object of framebuffer

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: 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. (cherry picked from commit 73dd0ad9e5dad53766ea3e631303430116f834b3)

References

• https://git.kernel.org/stable/c/093ee72ed35c2338c87c26b6ba6f0b7789c9e14e
• https://git.kernel.org/stable/c/f6f5e39a3fe7cbdba190f42b28b40bdff03c8cf0
• https://git.kernel.org/stable/c/49e1b214f3239b78967c6ddb8f8ec47ae047b051
• https://git.kernel.org/stable/c/3b9a33235c773c7a3768060cf1d2cf8a9153bc37