[Intel-gfx] Modeset without a detected monitor on DDI platforms

[Damien Lespiau]

Here's a tentative patch that I unfortunately can't test on HSW because my dev
machine seems quite unhappy. The patch had limited testing on simulation
though.

The general problem is that intel_digital_port encoders personalities (ie
intel_encoder->type) are only set by ->detect() at the moment. It'd be nice to
be able to set it at modeset time so any path that forces a modeset with a
specific connector end up with a correct encoder type.

-- 
Damien