drm/i915/guc: Split GuC firmware xfer function into clear steps

drm/i915/guc: Split GuC firmware xfer function into clear steps

Transfer of GuC firmware requires few steps that currently
are implemented in two large functions. Split this code into
smaller functions to make these steps small and clear.
Also be prepared for potential DMA xfer step failure.

v2: rename function steps (Sagar)

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/20171103151816.62048-1-michal.wajdeczko@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>