drm/v3d: Remove unused function header

v3d_mmu_get_offset header was added but the function was never defined.
Just remove it.

Signed-off-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231130164420.932823-3-mcanal@igalia.com