drm/i915/display: add generic to_intel_display() macro

drm/i915/display: add generic to_intel_display() macro

Convert various pointers to struct intel_display * using _Generic().

Add some macro magic to make adding new conversions easier, and somewhat
abstract the need to cast each generic association. The cast is required
because all associations needs to compile, regardless of the type and
the generic selection.

The use of *p in the generic selection assignment expression removes the
need to add separate associations for const pointers.

Note: This intentionally does *not* cover struct drm_i915_private or
struct xe_device. They are not to be used in the long run, so avoid
using this macro for them.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/02cf407961200db4379370856c779ea62b3eaa90.1713358679.git.jani.nikula@intel.com