drm/i915: Use __intel_uncore_wait_for_register_fw for sandybride_pcode_read

Since the sandybridge_pcode_read() may be called from
skl_pcode_request() inside an atomic context (with preempt disabled), we
should avoid hitting any sleeping paths. Currently is being called with
a 500ms timeout, irrespective of being inside an atomic context or not.
This is reduced down to 500us to play nice with the atomic context, and
that appears to be sufficient to keep BAT happy (we have a DRM_ERROR
should it timeout), i.e. we do not see any 500us pcode timeouts for
normal use. So leave it as a pure spin without having to introduce new
code paths to separate atomic/normal contexts.

Signed-off-by: Chris Wilson <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Tvrtko Ursulin <[email protected]>

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