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@vger.kernel.org # 4.19
Reported-by: S, Shirish <Shirish.S@amd.com>
Reported-by: Matthias Kaehlcke <mka@google.com>
Suggested-by: James Y Knight <jyknight@google.com>
Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Matthias Kaehlcke <mka@chromium.org>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>