drm/gma500: Fix SDVO command debug printing

At some point the DRM printers started adding a newline after each
print. This caused SDVO command debug printing to look weird. Fix this
by using snprintf to print into a buffer which can be printed as a whole
by DRM_DEBUG_KMS(). Code is heavily inspired by i915.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220610130925.8650-1-patrik.r.jakobsson@gmail.com