drm/i915: Remove 'prefault_disable' modparam

The 'prefault_disable' modparam was used by IGT to prevent a few
prefaulting operations to make fault handling under struct_mutex more
prominent. With the removal of struct_mutex, this is not as important
any more and we have almost completely stopped using the parameter. The
remaining use in execbuf is now immaterial and can be dropped without
affecting coverage.

We must re-address the idea of fault injection though.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200124230656.687503-1-chris@chris-wilson.co.uk