drm: remove no longer needed VISIBILITY_CFLAGS

With earlier commits we've annotated the private symbols, thus
we no longer require the -fvisibility=hidden CFLAGS.

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Damien Lespiau <damien.lespiau@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Cc: Michel Dänzer <michel.daenzer@amd.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>