drm/amdgpu: enable -msse2 for GCC 7.1+ users

drm/amdgpu: enable -msse2 for GCC 7.1+ users

A final attempt at enabling sse2 for GCC users.

Orininally attempted in:
commit 8d73ceda10c9 ("drm/amd/display: add -msse2 to prevent Clang from emitting libcalls to undefined SW FP routines")

Reverted due to "reported instability" in:
commit 4a3058cdd203 ("Revert "drm/amd/display: add -msse2 to prevent Clang from emitting libcalls to undefined SW FP routines"")

Re-added just for Clang in:
commit 4ba6fcf75364 ("drm/amd/display: readd -msse2 to prevent Clang from emitting libcalls to undefined SW FP routines")

The original report didn't have enough information to know if the GPF
was due to misalignment, but I suspect that it was. (The missing
information was the disassembly of the function at the bottom of the
trace, to see if the instruction pointer pointed to an instruction with
16B alignment memory operand requirements.  The stack trace does show
the stack was only 8B but not 16B aligned though, which makes this a
strong possibility).

Now that the stack misalignment issue has been fixed for users of GCC
7.1+, reattempt adding -msse2. This matches Clang.

It will likely never be safe to enable this for pre-GCC 7.1 AND use a
16B aligned stack in these translation units.

This is only a functional change for GCC 7.1+ users, and should be boot
tested.

Link: https://bugs.freedesktop.org/show_bug.cgi?id=109487
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>