drm/i915: use FPGA_DBG for the "unclaimed register" checks

We plan to treat GEN7_ERR_INT as an interrupt, so use this register
for the checks inside I915_WRITE. This way we can have the best of
both worlds: the error message with a register address and the

V2: Split in 2 patches: one for the macro, one for changing the
register, as requested by Ben.
V3: Rebase.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>