drm/kms/fb: move to using fb helper crtc grouping instead of core crtc list

This move to using the list of crtcs in the fb helper and cleans up the
whole picking code, now we store the crtc/connectors we want directly
into the modeset and we use the modeset directly to set the mode.

Fixes from James Simmons and Ben Skeggs.

Signed-off-by: Dave Airlie <airlied@redhat.com>