drm/panfrost: Move gpu_{write, read}() macros to panfrost_regs.h

So they can be used from other files.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190618081648.17297-2-boris.brezillon@collabora.com