drm/xe/uapi: Rename query's mem_usage to mem_regions

drm/xe/uapi: Rename query's mem_usage to mem_regions

'Usage' gives an impression of telemetry information where someone
would query to see how the memory is currently used and available
size, etc. However this API is more than this. It is about a global
view of all the memory regions available in the system and user
space needs to have this information so they can then use the
mem_region masks that are returned for the engine access.

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