drm/amd/display: Fix missing declaration of enable_kernel_vsx()

Include <asm/switch-to.h> in order to avoid following build failure
because of missing declaration of enable_kernel_vsx()

  CC [M]  drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o
In file included from ./drivers/gpu/drm/amd/amdgpu/../display/dc/dm_services_types.h:29,
                 from ./drivers/gpu/drm/amd/amdgpu/../display/dc/dm_services.h:37,
                 from drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:27:
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c: In function 'dcn_bw_apply_registry_override':
./drivers/gpu/drm/amd/amdgpu/../display/dc/os_types.h:64:3: error: implicit declaration of function 'enable_kernel_vsx'; did you mean 'enable_kernel_fp'? [-Werror=implicit-function-declaration]
   64 |   enable_kernel_vsx(); \
      |   ^~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:640:2: note: in expansion of macro 'DC_FP_START'
  640 |  DC_FP_START();
      |  ^~~~~~~~~~~
./drivers/gpu/drm/amd/amdgpu/../display/dc/os_types.h:75:3: error: implicit declaration of function 'disable_kernel_vsx'; did you mean 'disable_kernel_fp'? [-Werror=implicit-function-declaration]
   75 |   disable_kernel_vsx(); \
      |   ^~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.c:676:2: note: in expansion of macro 'DC_FP_END'
  676 |  DC_FP_END();
      |  ^~~~~~~~~
cc1: some warnings being treated as errors
make[5]: *** [drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dcn_calcs.o] Error 1

Fixes: 16a9dea ("amdgpu: Enable initial DCN support on POWER")
Cc: stable@vger.kernel.org
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>

drivers/gpu/drm/amd/display/dc/os_types.h

@@ -33,6 +33,7 @@
 #include <linux/slab.h>
 
 #include <asm/byteorder.h>
+#include <asm/switch-to.h>
 
 #include <drm/drm_print.h>