drm/savage: drop use of drm_os_linux

Drop use of macros from the deprecated drm_os_linux header.
Simple 1:1 replacements of
- DRM_UDELAY
- DRM_CURRENTPID
- DRM_READ
- DRM_WRITE

With this change we do not need to introduce the deprecated
drm_os_linux.h header when we drop use of drmP.h.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Link: https://patchwork.freedesktop.org/patch/msgid/20190605140313.13629-2-sam@ravnborg.org