drm/xe/uapi: Align on a common way to return arrays (engines)

The uAPI provides queries which return arrays of elements. As of now
the format used in the struct is different depending on which element
is queried. Fix this for engines by applying the pattern below:

        struct drm_xe_query_Xs {
           __u32 num_Xs;
           struct drm_xe_X Xs[];
           ...
        }

Instead of directly returning an array of struct
drm_xe_query_engine_info, a new struct drm_xe_query_engines is
introduced. It contains itself an array of struct drm_xe_engine
which holds the information about each engine.

v2: Use plural for struct drm_xe_query_engines as multiple engines
    are returned (José Roberto de Souza)

Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>