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>