drm: Add drm_puts() to complement drm_printf()

Add drm_puts() for a much faster path to print constant strings
into a drm_printer object with memcpy and friends. This can
have seconds off of really large outputs such as GPU dumps.

If the drm_printer object supports a custom puts function then
use that otherwise fall back to the slower legacy printf call.

v2: Add documentation for drm_puts() per Daniel Vetter

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
[robclark fix minor htmldocs warning]
Signed-off-by: Rob Clark <robdclark@gmail.com>

include/drm/drm_print.h

@@ -69,6 +69,7 @@
 struct drm_printer {
 	/* private: */
 	void (*printfn)(struct drm_printer *p, struct va_format *vaf);
+	void (*puts)(struct drm_printer *p, const char *str);
 	void *arg;
 	const char *prefix;
 };
@@ -80,6 +81,7 @@ void __drm_printfn_debug(struct drm_printer *p, struct va_format *vaf);
 
 __printf(2, 3)
 void drm_printf(struct drm_printer *p, const char *f, ...);
+void drm_puts(struct drm_printer *p, const char *str);
 
 __printf(2, 0)
 /**