Add option to specify engine for register read/write operation.
If engine is specified, use MI_LOAD_REGISTER_IMM and MI_STORE_REGISTER_IMM
to write and read register using a batch targeted at that engine.

v2: no MI_NOOP after BBE (Chris)
v3: use modern engine names (Chris), use global fd
v4: strcasecmp (Chris)
v5: use register definition format for engine (Jani)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
CC: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk> (v4)
Acked-by: Jani Nikula <jani.nikula@intel.com>