drm/i915: adjust get_crtc_fence_y_offset() to use base.y instead of crtc.y

This is to use clipped y coordinate here. I left get_crtc_fence_y_offset()
function itself in place as oneliner to maintain comment above it why this
is done.

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/1508270891-22186-6-git-send-email-juhapekka.heikkila@gmail.com