drm/amd/pm: avoid large variable on kernel stack

The activity_monitor_external[] array is too big to fit on the
kernel stack, resulting in this warning with clang:

drivers/gpu/drm/amd/amdgpu/../pm/swsmu/smu13/smu_v13_0_7_ppt.c:1438:12: error: stack frame size (1040) exceeds limit (1024) in 'smu_v13_0_7_get_power_profile_mode' [-Werror,-Wframe-larger-than]

Use dynamic allocation instead. It should also be possible to
have single element here instead of the array, but this seems
easier.

v2: fix up argument to sizeof() (Alex)

Fixes: 334682ae ("drm/amd/pm: enable workload type change on smu_v13_0_7")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>