drm/i915: drop alpha_support for good in favour of force_probe

It's been a long enough transition period since the DRM_I915_FORCE_PROBE
config and i915.force_probe module parameter were introduced in commit
7ef5ef5c ("drm/i915: add force_probe module parameter to replace
alpha_support"). Remove alpha support.

Cc: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121103020.26494-1-jani.nikula@intel.com