drm: armada: Cleanup drm_display_mode print str

This patch adjust the print string of drm_display_mode object
to remove drm_mode_object dependency in armada files.

Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/dddd98f1a6687a37444d315adc4cbd8a692a8131.1547214023.git.shayenneluzmoura@gmail.com