drm/i915: Remove useless error return from intel_init_mocs_engine()

As the only error is for a programming error in constructing the static
tables describing the register values, replace the error code
propagation with an assert.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180815184251.5850-1-chris@chris-wilson.co.uk