drm/i915: Kconfig option to disable the legacy fbdev support

Boots Just Fine (tm)!

The only glitch seems to be that at least on Fedora the boot splash
gets confused and doesn't display much at all.

And since there's no ugly console flickering anymore in between, the
flicker while switching between X servers (VT support is still enabled)
is even more jarring.

Also, I'm unsure whether we don't need to somehow kick out vgacon, now
that nothing else gets in the way. But stuff seems to work, so I
don't care. Also everything still works as well with VGA_CONSOLE=n

Also the #ifdef mess needs a bit of a cleanup, follow-up patches will
do just that.

To keep the Kconfig tidy, extract all the i915 options into its own
file.

v2:
- Rebase on top of the preliminary hw support option and the
  intel_drv.h cleanup.
- Shut up warnings in i915_debugfs.c

v3: Use the right CONFIG variable, spotted by Chon Ming.

Cc: Lee, Chon Ming <chon.ming.lee@intel.com>
Cc: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Chon Ming Lee <chon.ming.lee@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>