drm/i915: remove last traces of I915_READ_FW() and I915_WRITE_FW()

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

intel_uncore_read_fw() and intel_uncore_write_fw() should be used
instead.

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201130111601.2817-2-jani.nikula@intel.com