regression in dcn linked to -msse2

[StDenis, Tom]

Testing with the new 5.0.0-rc1 amd-staging-drm-next branch results in 
this commit causing the attached lockup on init with my Raven + Polaris 
system:

10117450735c7a7c0858095fb46a860e7037cb9a is the first bad commit
commit 10117450735c7a7c0858095fb46a860e7037cb9a
Author: ndesaulniers at google.com <ndesaulniers at google.com>
Date:   Thu Jan 24 16:52:59 2019 -0800

     drm/amd/display: add -msse2 to prevent Clang from emitting libcalls 
to undefined SW FP routines

     arch/x86/Makefile disables SSE and SSE2 for the whole kernel.  The
     AMDGPU drivers modified in this patch re-enable SSE but not SSE2.  Turn
     on SSE2 to support emitting double precision floating point 
instructions
     rather than calls to non-existent (usually available from gcc_s or
     compiler_rt) floating point helper routines.

     Link: 
https://gcc.gnu.org/onlinedocs/gccint/Soft-float-library-routines.html
     Link: https://github.com/ClangBuiltLinux/linux/issues/327
     Cc: stable at vger.kernel.org # 4.19
     Reported-by: S, Shirish <Shirish.S at amd.com>
     Reported-by: Matthias Kaehlcke <mka at google.com>
     Suggested-by: James Y Knight <jyknight at google.com>
     Suggested-by: Nathan Chancellor <natechancellor at gmail.com>
     Signed-off-by: Nick Desaulniers <ndesaulniers at google.com>
     Tested-by: Guenter Roeck <linux at roeck-us.net>
     Tested-by:  Matthias Kaehlcke <mka at chromium.org>
     Tested-by: Nathan Chancellor <natechancellor at gmail.com>
     Reviewed-by: Harry Wentland <harry.wentland at amd.com>
     Signed-off-by: Harry Wentland <harry.wentland at amd.com>
     Signed-off-by: Alex Deucher <alexander.deucher at amd.com>

:040000 040000 39f89ea177b8a69a3a2aa098aef8b1a7e58056b9 
71edb262be0608ffae0996253f47492b2e714b5c M      drivers


Cheers,
Tom
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