drm/i915: Remove obsolete code from intelfb_alloc()

drm/i915: Remove obsolete code from intelfb_alloc()

Clean up after 78639275bfee ("drm/i915: Pin the ifbdev for the
info->system_base GGTT mmapping"):

At each of the remaining "goto out" in intelfb_alloc(), fb can only be
either an ERR_PTR or NULL, so the call to drm_framebuffer_unreference()
is now obsolete.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: http://patchwork.freedesktop.org/patch/msgid/56756c41.c306c20a.d0602.1830SMTPIN_ADDED_MISSING@mx.google.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>