drm/amd/pm: fix null pointer access

Writing a string without delimiters (' ', '\n', '\0') to the under
gpu_od/fan_ctrl sysfs or pp_power_profile_mode for the CUSTOM profile
will result in a null pointer dereference.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4401


Signed-off-by: Umio Yasuno <coelacanth_dream@protonmail.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>