anv/gpu_memcpy: Rename the gpu_memcpy function

anv/gpu_memcpy: Rename the gpu_memcpy function

A GPU memcpy function could alternatively be implemented using MI_*
commands. Provide more detail into how this one operates in case another
memcpy function is created.

v2:
- Update the commit message.
v3:
- Use 'memcpy' instead of 'cpy' (Jason Ekstrand)
- Shorten 'streamout' to 'so'

Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com> (v2)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>