drm/i915/query: Split out query item checks

This simplifies adding new query item objects.

v2: Use query_hdr (Tvrtko, Chris).
    int instead of u32 in return (Tvrtko)
v3: More naming fixes (Tvrtko)

Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190211173251.7131-1-abdiel.janulgue@linux.intel.com