drm/i915/gt: use engine instance directly for offset

To make it uniform across copy and clear, use the engine offset directly
to calculate the offset in the cmd forming for emit_clear.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Thomas Hellstrom <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220405150840.29351-2-ramalingam.c@intel.com