drm/i915: Enable -Wsometimes-uninitialized

This warning helps catch uninitialized variables. It should have been
enabled at the same time as commit b2423184 ("drm/i915: Enable
-Wuninitialized") but I did not realize they were disabled separately.
Enable it now that i915 is clean so that it stays that way.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210824225427.2065517-4-nathan@kernel.org