drm/i915: Record GT workarounds in a list

To enable later verification of GT workaround state at various stages of
driver lifetime, we record the list of applicable ones per platforms to a
list, from which they are also applied.

The added data structure is a simple array of register, mask and value
items, which is allocated on demand as workarounds are added to the list.

This is a temporary implementation which later in the series gets fused
with the existing per context workaround list handling. It is separated at
this stage since the following patch fixes a bug which needs to be as easy
to backport as possible.

Also, since in the following patch we will be adding a new class of
workarounds (per engine) which can be applied from interrupt context, we
straight away make the provision for safe read-modify-write cycle.

v2:
 * Change dev_priv to i915 along the init path. (Chris Wilson)
 * API rename. (Chris Wilson)

v3:
 * Remove explicit list size tracking in favour of growing the allocation
   in power of two chunks. (Chris Wilson)

v4:
 Chris Wilson:
 * Change wa_list_finish to early return.
 * Copy workarounds using the compiler for static checking.
 * Do not bother zeroing unused entries.
 * Re-order struct i915_wa_list.

v5:
 * kmalloc_array.
 * Whitespace cleanup.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203133319.10174-1-tvrtko.ursulin@linux.intel.com