drm/i915: Use bool i915_param.alpha_support

The alpha_support module option can only take one of two values, so
assign it to a boolean type. The only advantage is in pretty printing
via /sys/module/i915/parameters/alpha_support and elsewhere.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170206213608.31328-2-chris@chris-wilson.co.uk