drm: remove drm_public macro

Some compilers (like the Oracle Studio), require that the function
declaration must be annotated with the same visibility attribute as the
definition. As annotating functions with drm_public is no longer
required just remove the macro.

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>