drm/i915: use mappable size for fb kickout

The GTT start is either 0 in the KMS case, or some value which is set
only after the init IOCTL in the UMS case. In both cases, we don't have
this information until after we've tried to kick out the firmware fb.

This patch should have no functional change since we kzalloc the GTT
struct anyway. It only clarifies the situation for people who end up
having to look at that code.

This weirdness was introduced in:

commit 93d18799
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Thu Jan 17 12:45:17 2013 -0800

    drm/i915: Remove use of gtt_mappable_entries

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>