drm/i915: Move a bunch of workaround-related code to its own file

This has grown to be a sizable amount of code, so move it to
its own file before we try to refactor anything. For the moment,
we are leaving behind the WA BB code and the WAs that get applied
(incorrectly) in init_clock_gating, but we will deal with it later.

v2: Use intel_ prefix for code that deals with the hardware (Chris)
v3: Rebased
v4:
  - Rebased
  - New license header
v5:
  - Rebased
  - Added some organisational notes to the file (Chris)
v6: Include DOC section in the documentation build (Jani)

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[ickle: appease checkpatch, mostly]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/1523376767-18480-1-git-send-email-oscar.mateo@intel.com