drm/printer: Add drm_vprintf()

Simple va_args equivalent to the existing drm_printf() for use with the
drm_printer.

v2: Fixup kerneldoc to match final parameter names.
v3: Turn it into a kerneldoc comment

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123084051.30203-1-chris@chris-wilson.co.uk