drm/i915: Enable -Wuninitialized

-Wunintialized was disabled in commit c5627461 ("drm/i915: Disable
-Wuninitialized") because there were two warnings that were false
positives. The first was due to DECLARE_WAIT_QUEUE_HEAD_ONSTACK, which
was fixed in LLVM 9.0.0. The second was in busywait_stop, which was
fixed in LLVM 10.0.0 (issue 415). The kernel's minimum version for LLVM
is 10.0.1 so this warning can be safely enabled, where it has already
caught a couple bugs.

Link: https://github.com/ClangBuiltLinux/linux/issues/220
Link: https://github.com/ClangBuiltLinux/linux/issues/415
Link: https://github.com/ClangBuiltLinux/linux/issues/499
Link: https://github.com/llvm/llvm-project/commit/2e040398f8d691cc378c1abb098824ff49f3f28f
Link: https://github.com/llvm/llvm-project/commit/c667cdc850c2aa821ffeedbc08c24bc985c59edd
Fixes: c5627461 ("drm/i915: Disable -Wuninitialized")
References: 2ea4a7ba ("drm/i915/gt: Avoid uninitialized use of rpcurupei in frequency_show")
References: 2034c212 ("drm/i915/display: Ensure that ret is always initialized in icl_combo_phy_verify_state")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210216212953.24458-1-nathan@kernel.org

drivers/gpu/drm/i915/Makefile

@@ -21,7 +21,6 @@ subdir-ccflags-y += $(call cc-disable-warning, unused-but-set-variable)
 subdir-ccflags-y += $(call cc-disable-warning, sign-compare)
 subdir-ccflags-y += $(call cc-disable-warning, sometimes-uninitialized)
 subdir-ccflags-y += $(call cc-disable-warning, initializer-overrides)
-subdir-ccflags-y += $(call cc-disable-warning, uninitialized)
 subdir-ccflags-y += $(call cc-disable-warning, frame-address)
 subdir-ccflags-$(CONFIG_DRM_I915_WERROR) += -Werror