drm/i915: add force_probe module parameter to replace alpha_support

The i915.alpha_support module parameter has caused some confusion along
the way. Add new i915.force_probe parameter to specify PCI IDs of
devices to probe, when the devices are recognized but not automatically
probed by the driver. The name is intended to reflect what the parameter
effectively does, avoiding any overloaded semantics of "alpha" and
"support".

The parameter supports "" to disable, "<pci-id>,[<pci-id>,...]" to
enable force probe for one or more devices, and "*" to enable force
probe for all known devices.

Also add new CONFIG_DRM_I915_FORCE_PROBE config option to replace the
DRM_I915_ALPHA_SUPPORT option. This defaults to "*" if
DRM_I915_ALPHA_SUPPORT=y.

Instead of replacing i915.alpha_support immediately, let the two coexist
for a while, with a deprecation message, for a transition period.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190506134801.28751-1-jani.nikula@intel.com