drm/drm_blend: Remove usage of deprecated DRM_DEBUG_ATOMIC

drm_print.h says DRM_DEBUG_ATOMIC is deprecated in favor of
drm_dbg_atomic().

Signed-off-by: Siddh Raman Pant <code@siddh.me>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/1a5b5aa012689572baf561eea0dd0b5934645af5.1673269059.git.code@siddh.me