drm: drm_printer: add __printf validation

drm_printf does not currently use the compiler to verify
format and arguments.  Make it do so.

Miscellanea:

o Add appropriate #include files for __printf and struct va_format
o Convert dev_printk to dev_info

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/133858f214e9b90f92bb8eb44c6b1dc04429933d.1487201526.git.joe@perches.com