drm/amdgpu: check ras and obj before dereference

nbio_v7_9_handle_ras_controller_intr_no_bifring() dereferences ras and obj
without checking either for NULL. Both amdgpu_ras_get_context() and
amdgpu_ras_find_obj() can return NULL, e.g. during the window between
adev->nbio.ras being set (early in amdgpu_ras_init(), by design, to
enable the fatal-error interrupt as soon as possible) and the PCIE_BIF
ras object actually being created in RAS late_init. Any interrupt in that
window crashes in hard-IRQ context.

This is analogous to commit d190b459b2a4 ("drm/amdgpu: the warning
dereferencing obj for nbio_v7_4"), which fixed the same issue in the
nbio_v7_4 handler.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 7692e1ee2446 ("drm/amdgpu: add RAS fatal error handler for NBIO v7.9")
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Dmitriy Chumachenko <Dmitry.Chumachenko@cyberprotect.ru>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit c7071767a50a32ed727cf800ac84372429e3b4b3)