anv/gpu_memcpy: Add a lighter-weight GPU memcpy function

anv/gpu_memcpy: Add a lighter-weight GPU memcpy function

We'll be performing a GPU memcpy in more places to copy small amounts of
data. Add an alternate function that thrashes less state.

v2:
- Make a new function (Jason Ekstrand).
- Move the #define into the function.
v3:
- Update the function name (Jason).
- Update comments.
v4: Use an indirect drawing register as TEMP_REG (Jason Ekstrand).

Signed-off-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>