drm/i915/gtt: Mark ALL_ENGINES as dirty on ppGTT modification

Small simplification to set all bits in the dirty mask rather than
lookup the exact mask of populated engines. The bits for the engines
that do not exist are unused and so can safely set and then ignored.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305180332.30900-2-chris@chris-wilson.co.uk