drm/selftests: Rename the Kconfig option to CONFIG_DRM_DEBUG_SELFTEST

drm/selftests: Rename the Kconfig option to CONFIG_DRM_DEBUG_SELFTEST

We want to add more DRM selftests, and there's not much point in
having a Kconfig option for every single one of them, so make
a generic one.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503112217.37292-5-maarten.lankhorst@linux.intel.com
[mlankhorst: Fix i915/Kconfig.debug (ickle)]
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>