drm/amd/ras: Add NULL checks for ras_core sys_fn callbacks

drm/amd/ras: Add NULL checks for ras_core sys_fn callbacks

Some ras core helper functions access ras_core and its callback
table (sys_fn) without validating them first.

Cc: Tao Zhou <tao.zhou1@amd.com>
Cc: YiPeng Chai <YiPeng.Chai@amd.com>
Cc: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: YiPeng Chai <YiPeng.Chai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>