drm/i915: Ditch INTELFB_CONN_LIMIT

And the gratious overallocation of crtcs. Seems to go back to the ums
days of yonder ...

We also still need it to make the fbdev emulation happy, but I don't
think there's really a need. Especially since the current fbdev
emulation doesn't actually support cloning.

v2: Use sizeof(*pointer) pattern (Jani).

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>