drm: omapdrm: Cleanup drm_display_mode print str

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

Signed-off-by: Shayenne Moura <shayenneluzmoura@gmail.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/cb6079fa6de6fda8d865a1d2a61d7cf10019ae88.1545308167.git.shayenneluzmoura@gmail.com