drm/i915/query: Protect tainted function pointer lookup

Smatch identifies i915_query_ioctl() as being a potential victim of
Spectre due to its use of a tainted user index into a function pointer
array. Use array_index_nospec() to defang the user index before using it
to lookup the function pointer.

Fixes: a446ae2c ("drm/i915: add query uAPI")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180521210530.26008-1-chris@chris-wilson.co.uk