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
7ef5ef5cde ("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

drivers/gpu/drm/i915/i915_pci.c

@@ -928,13 +928,6 @@ static bool force_probe(u16 device_id, const char *devices)
 	char *s, *p, *tok;
 	bool ret;
 
-	/* FIXME: transitional */
-	if (i915_modparams.alpha_support) {
-		DRM_INFO("i915.alpha_support is deprecated, use i915.force_probe=%04x instead\n",
-			 device_id);
-		return true;
-	}
-
 	if (!devices || !*devices)
 		return false;