drm/i915/guc: Drop legacy workarounds from guc_prepare_xfer

drm/i915/guc: Drop legacy workarounds from guc_prepare_xfer

We don't keep the workarounds for pre-production hardware
(see intel_detect_preproduction_hw) thus we can drop some
extra steps during firmware upload needed only for unsupported
platforms.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171103151816.62048-3-michal.wajdeczko@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>