drm/i915: rename preliminary_hw_support to alpha_support

The term "preliminary hardware support" has always caused confusion both
among users and developers. It has always been about preliminary driver
support for new hardware, and not so much about preliminary hardware. Of
course, initially both the software and hardware are in early stages,
but the distinction becomes more clear when the user picks up production
hardware and an older kernel to go with it, with just the early support
we had for the hardware at the time the kernel was released. The user
has to specifically enable the alpha quality *driver* support for the
hardware in that specific kernel version.

Rename preliminary_hw_support to alpha_support to emphasize that the
module parameter, config option, and flag are about software, not about
hardware. Improve the language in help texts and debug logging as well.

This appears to be a good time to do the change, as there are currently
no platforms with preliminary^W alpha support.

Cc: Rob Clark <robdclark@gmail.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477909108-18696-1-git-send-email-jani.nikula@intel.com