drm/i915: Avoid truncation before clamping userspace's priority value

Userspace provides a 64b value for the priority, we need to be careful
to preserve the full range before validation to prevent truncation (and
letting an illegal value pass).
Reported-by: Antonio Argenziano <antonio.argenziano@intel.com>
Fixes: ac14fbd4 ("drm/i915/scheduler: Support user-defined priorities")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Antonio Argenziano <antonio.argenziano@intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180208085151.11480-1-chris@chris-wilson.co.ukReviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -804,7 +804,7 @@ int i915_gem_context_setparam_ioctl(struct drm_device *dev, void *data,
 
 	case I915_CONTEXT_PARAM_PRIORITY:
 		{
-			int priority = args->value;
+			s64 priority = args->value;
 
 			if (args->size)
 				ret = -EINVAL;