drm/i915: clear FPGA_DBG_RM_NOCLAIM when capturing error state

In the error state function we read the registers without checking if
the power well is on, so after doing this we have to clear the
FPGA_DBG_RM_NOCLAIM bit to prevent the next I915_WRITE from detecting
it and printing an error message.

The first version of this patch was checking for the power well state
and then avoiding reading registers that were off, but the reviewers
requested to just read the registers any way and then later clear the
FPGA_DBG_RM_NOCLAIM bit.

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