drm/i915: dspaddr_offset doesn't need to be more than local variable

Move u32 dspaddr_offset from struct intel_crtc member into local
variable in i9xx_update_primary_plane()

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1508270891-22186-3-git-send-email-juhapekka.heikkila@gmail.com