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>

drivers/gpu/drm/amd/display/dc/calcs/Makefile

@@ -30,7 +30,7 @@ else ifneq ($(call cc-option, -mstack-alignment=16),)
 	cc_stack_align := -mstack-alignment=16
 endif
 
-calcs_ccflags := -mhard-float -msse $(cc_stack_align)
+calcs_ccflags := -mhard-float -msse -msse2 $(cc_stack_align)
 
 CFLAGS_dcn_calcs.o := $(calcs_ccflags)
 CFLAGS_dcn_calc_auto.o := $(calcs_ccflags)

drivers/gpu/drm/amd/display/dc/dml/Makefile

@@ -30,7 +30,7 @@ else ifneq ($(call cc-option, -mstack-alignment=16),)
 	cc_stack_align := -mstack-alignment=16
 endif
 
-dml_ccflags := -mhard-float -msse $(cc_stack_align)
+dml_ccflags := -mhard-float -msse -msse2 $(cc_stack_align)
 
 CFLAGS_display_mode_lib.o := $(dml_ccflags)
 CFLAGS_display_pipe_clocks.o := $(dml_ccflags)