drm/i915/fbc: don't store the fb_id on reg_params

We don't actually use fb_id anywhere. We already compare all
parameters that matter to the hardware: pixel format, stride,
fence_reg and ggtt_offset. The ID shouldn't make a difference.

Besides, we already update the FBC data at every modeset/flip, so this
can't change behind our backs.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-23-git-send-email-paulo.r.zanoni@intel.com