The DRM_READ, DRM_WRITE macros comes from the deprecated drm_os_linux.h
header file. Remove their use to remove this dependency.

Replace the use of the macros with static inline variants.

v4:
- Use a more standard via_write8_mask() function (Emil)

v3:
- Use static inline functions, rather than macros (Emil)
- Use dedicated mask variants for byte access (Emil)
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Kevin Brace <kevinbrace@gmx.com>
Cc: Thomas Hellstrom <thellstrom@vmware.com>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Mike Marshall <hubcap@omnibond.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Michel Dänzer <michel@daenzer.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723200944.17285-2-sam@ravnborg.org