drm/i915/cnl: Remove alpha_support protection

drm/i915/cnl: Remove alpha_support protection

We now have a stable cnl on our CI and it seems mostly
green without big risks of blank screen or anything
blowing up on linux installations in the future.

As a reminder i915.alpha_support was created to protect
future linux installation's iso images that might contain a
kernel from the enabling time of the new platform. Without this
protection most of linux installation was recommending
nomodeset option during installation that was getting stick
there after installation.

Specifically, alpha support says nothing about the development
state of the hardware, and everything about the state of the
driver in a kernel release.

This is semantically no different from the old
preliminary_hw_support flag, but the old one was all too often
interpreted as (preliminary hw) support instead of the intended
(preliminary) hw support, and it was misleading for everyone.
Hence the rename.

v2: Fix the typos and include more history about the parameter
rename on commit message. (Jani)

Reference: https://intel-gfx-ci.01.org/tree/drm-tip/fi-cnl-y3.html
Cc: James Ausmus <james.ausmus@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Jani Saarinen <jani.saarinen@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214204205.4446-1-rodrigo.vivi@intel.com