drm/i915: Delay disabling of VGA memory until vgacon->fbcon handoff is done

When transitioning away from vgacon the system tries to save the
current contents of the VGA memory, so that it can be cleanly handed
off to fbcon (or whatever comes afterwards).

The recent change

 commit 81b5c7bc
 Author: Alex Williamson <alex.williamson@redhat.com>
 Date:   Wed Aug 28 09:39:08 2013 -0600

    i915: Update VGA arbiter support for newer devices

caused i915 to disable VGA memory decode for the IGD when i915 is
initializing. Unfortunately that happens before the vgacon->fbcon
handoff so vgacon_save_screen() will read out all ones from the
VGA memory.

After the handoff fbcon will inherit the bogus state from vgacon,
and pre-fills the fb with matching contents. The end result is
a white rectangle in the top left corner of the screen, the size
of which matches the now inactive VGA console.

To remedy the situation delay the disabling of VGA memory until
the vgacon->fbcon handoff has happened.

Also rename i915_enable_vga to i915_enable_vga_mem to make
the relationship between these functions clearer.

Cc: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>