drm/i915: Fix potential spectre vulnerability

Fix smatch warning:
drivers/gpu/drm/i915/gem/i915_gem_context.c:847 set_proto_ctx_sseu()
warn: potential spectre issue 'pc->user_engines' [r] (local cap)

Fixes: d4433c7 ("drm/i915/gem: Use the proto-context to handle create parameters (v5)")
Cc: <stable@vger.kernel.org> # v5.15+
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103110922.430122-1-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/gem/i915_gem_context.c

@@ -844,6 +844,7 @@ static int set_proto_ctx_sseu(struct drm_i915_file_private *fpriv,
 		if (idx >= pc->num_user_engines)
 			return -EINVAL;
 
+		idx = array_index_nospec(idx, pc->num_user_engines);
 		pe = &pc->user_engines[idx];
 
 		/* Only render engine supports RPCS configuration. */