drm/i915: Disregard drm_mode_config.fb_base

The fb_base is only used for communicating the GTT BAR from one piece of
the display code (kms setup) to another (fbdev). What is required in the
fbdev is just the aperture address which should be derived from the
bo we allocate for the framebuffer directly.

The same appears true for drm/; it is not used by the core or the uAPI,
it is merely for conveniently passing a device address from bit of
display management code to another.

v2: Note that since we only expose enough of a system map to cover our
single framebuffer, the screen_base/size and the smem are one and the
same.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813182112.23227-1-chris@chris-wilson.co.uk