drm/amdgpu: Declare array with strings as pointers constant

This warning is for the declaration of a static array, and it is
recommended to declare it as type "static const char * const" instead of
"static const char *".

an array pointer declared as type "static const char *" can point to a
different character constant because the pointer is mutable. However, if
it is declared as type "static const char * const", the pointer will
point to an immutable character constant, preventing it from being
modified which can better ensure the safety and stability of the
program.

Fixes the below:

WARNING: static const char * array should probably be static const char * const

Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Pan, Xinhui" <Xinhui.Pan@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>