Mark functions printf-like where possible

These functions all take a format string and either a list of variable
arguments or a va_list. Use the new DRM_PRINTFLIKE macro to tell the
compiler about it so that the arguments can be checked against the
format string.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Thierry Reding <treding@nvidia.com>