drm/amd/display: add -msse2 to prevent Clang from emitting libcalls to undefined...

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: [email protected] # 4.19
Reported-by: S, Shirish <[email protected]>
Reported-by: Matthias Kaehlcke <[email protected]>
Suggested-by: James Y Knight <[email protected]>
Suggested-by: Nathan Chancellor <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Tested-by:  Matthias Kaehlcke <[email protected]>
Tested-by: Nathan Chancellor <[email protected]>
Reviewed-by: Harry Wentland <[email protected]>
Signed-off-by: Harry Wentland <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>