With i915 added to i915_inject_probe_failure we can use dedicated
printk when injecting artificial load failure.

Also make this function look like other i915 functions that return
error code and make it more flexible to return any provided error
code instead of previously assumed -ENODEV.
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190802184055.31988-2-michal.wajdeczko@intel.com