drm/loongson: use drm_gem_ttm_dumb_map_offset()

drm/loongson: use drm_gem_ttm_dumb_map_offset()

Replace the open-coded lsdc_dumb_map_offset() with the generic
drm_gem_ttm_dumb_map_offset() helper, which is functionally identical.
This removes unnecessary boilerplate and aligns with the DRM convention
for TTM-based drivers.

Signed-off-by: Amin GATTOUT <amin.gattout@gmail.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patch.msgid.link/20260308-master-v1-1-af32d71c8a1d@gmail.com