drm/vc4: Protect madv read in vc4_gem_object_mmap() with madv_lock

drm/vc4: Protect madv read in vc4_gem_object_mmap() with madv_lock

The mmap callback reads bo->madv without holding madv_lock, racing with
concurrent DRM_IOCTL_VC4_GEM_MADVISE calls that modify the field under
the same lock. Add the missing locking to prevent the data race.

Fixes: b9f19259b84d ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
Reviewed-by: Melissa Wen <mwen@igalia.com>
Link: https://patch.msgid.link/20260330-vc4-misc-fixes-v1-4-92defc940a29@igalia.com
Signed-off-by: Maíra Canal <mcanal@igalia.com>