drm/i915: remove last traces of I915_READ(), I915_WRITE() and POSTING_READ()

Good riddance! Remove the macros and their remaining references in
comments.

The following functions should be used instead, depending on the use
case:

- intel_uncore_read(), intel_uncore_write(), intel_uncore_posting_read()

- intel_de_read(), intel_de_write(), intel_de_posting_read()

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-10-jani.nikula@intel.com