drm: add helper for printing to log or seq_file

Sometimes it is nice not to duplicate equivalent printk() and
seq_printf() code.

v2: simplify things w/ va_format, and use dev_printk, docs

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1478358492-30738-3-git-send-email-robdclark@gmail.com