drm/i915/gt: fix platform prefix

gen8_clear_engine_error_register() is actually not used by
GRAPHICS_VER >= 8, since for those we are using another register that is
not engine-dependent. Fix the platform prefix, to make clear we are not
using any GEN6_RING_FAULT_REG_* one GRAPHICS_VER >= 8.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720232014.3302645-2-lucas.demarchi@intel.com