drm/i915/glk: Remove the alpha_support flag

Geminilake is now included in CI, making it part of the pre-merge
criteria. The support should be in good enough shape, so let's remove
the alpha_support flag.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170608114800.17201-1-ander.conselvan.de.oliveira@intel.com