As a result of commit 987d65d01356 (drm: debugfs: make
drm_debugfs_create_files() never fail) and changes to various debugfs
functions in drm/core and across various drivers, there is no need for
the drm_driver.debugfs_init() hook to have a return value. Therefore,
declare it as void.
This also includes refactoring all users of the .debugfs_init() hook to
return void across the subsystem.
v2: include changes to the hook and drivers that use it in one patch to
prevent driver breakage and enable individual successful compilation of
this change.