drm/amdgpu: Replace one-elements array with flex-array members

One-element arrays are deprecated, and we are replacing them with
flexible array members instead. So, replace one-element array with
flexible-array member in structs ATOM_I2C_VOLTAGE_OBJECT_V3,
ATOM_ASIC_INTERNAL_SS_INFO_V2, ATOM_ASIC_INTERNAL_SS_INFO_V3,
and refactor the rest of the code accordingly.

Important to mention is that doing a build before/after this patch
results in no functional binary output differences.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/238
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836

 [1]

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>