drm/i915/icl: Remove alpha support protection

drm/i915/icl: Remove alpha support protection

Now with the watermarks fixes merged, Icelake is stable enough to
have the alpha support protection flag removed.

We have a few ICL machines in our CI and it is mostly green with
failures in tests that will not impact future linux installations.
Also there is no warnings, errors, flickering or any visual defects
while doing ordinary tasks like browsing and editing documents in a
dual monitor setup.

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.

Reference: https://intel-gfx-ci.01.org/tree/drm-tip/fi-icl-y.html
Reference: https://intel-gfx-ci.01.org/tree/drm-tip/shard-iclb.html
Cc: James Ausmus <james.ausmus@intel.com>
Cc: Jani Saarinen <jani.saarinen@intel.com>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190305221153.359-1-jose.souza@intel.com