drm/i915: use localized __diag_ignore_all() instead of per file

Use localized __diag_push(), __diag_ignore_all() with rationale, and
__diag_pop() for specific initializations instead of blanket disabling
of -Woverride-init across several files.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230525210653.1048972-1-jani.nikula@intel.com