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 <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
[mlankhorst: Fix i915/Kconfig.debug (ickle)]
Acked-by: Chris Wilson <[email protected]>
Acked-by: Daniel Vetter <[email protected]>