drm/i915: Make psr_safest_params and enable_psr2_sel_fetch parameters read only

By mistake those 2 parameters were defined as read and write in the
.h file while in the .c file it is read only.
The intention here was to be read only to avoid the need of additional
handling.

Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Cc: Petri Latvala <petri.latvala@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210204153357.39681-1-jose.souza@intel.com