At least the i915 and amd drivers enable a bunch more compiler warnings
than the kernel defaults.

Extend most of the W=1 warnings to the entire drm subsystem by
default. Use the copy-pasted warnings from scripts/Makefile.extrawarn
with s/KBUILD_CFLAGS/subdir-ccflags-y/ to make it easier to compare and
keep up with them in the future.

This is similar to the approach currently used in i915.

Some of the -Wextra warnings do need to be disabled, just like in
Makefile.extrawarn, but take care to not disable them for W=2 or W=3
builds, depending on the warning.

There are too many -Wformat-truncation warnings to cleanly fix up front;
leave that warning disabled for now.

v3:
- Drop -Wmissing-declarations (already enabled by default)
- Drop -Wmissing-prototypes (already enabled by default)

v2:
- Drop -Wformat-truncation (too many warnings)
- Drop -Wstringop-overflow (already enabled by default)

Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Pan
Cc: Karol Herbst <kherbst@redhat.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Rob Clark <robdclark@gmail.com>
Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Cc: Sean Paul <sean@poorly.run>
Cc: Marijn Suijten <marijn.suijten@somainline.org>
Cc: Hamza Mahfooz <hamza.mahfooz@amd.com>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Acked-by: Danilo Krummrich <dakr@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/a50f1a69d5af72e913996179a75bc3a71d81ebea.1709629403.git.jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>