drm/i915: nuke local versions of WARN_ON/WARN_ON_ONCE

In general, we should avoid redefining kernel macros like this. It can
get confusing, and what gets used will depend on whether the header is
included or not. Moreover, we should prefer drm_WARN_ON() and
drm_WARN_ON_ONCE() anyway, which include the stringified error condition
in the message.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220121132957.3778555-1-jani.nikula@intel.com