drm/i915: use fine grained -Woverride-init disable

drm/i915: use fine grained -Woverride-init disable

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.

Note that we've tried this before with commit 88e9664434c9 ("drm/i915:
use localized __diag_ignore_all() instead of per file") and reverted in
commit 290d16104575 ("Revert "drm/i915: use localized
__diag_ignore_all() instead of per file""). The issue turned out to be
in __diag_ignore_all() and it was fixed by commit 689b097a06ba
("compiler-gcc: Suppress -Wmissing-prototypes warning for all supported
GCC"). So we should be able to pull this off now.

Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240328102455.944131-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>