main: memcpy larger chunks in _mesa_propagate_uniforms_to_driver_storage

main: memcpy larger chunks in _mesa_propagate_uniforms_to_driver_storage

When possible, do the memcpy on larger blocks. This reduces cycles
spent in _mesa_propagate_uniforms_to_driver_storage from
1.51 % to 0.62% according to perf during the Unigine Heaven benchmark.
It did not affect the framerate of the benchmark. The system used for
testing was an i5 6600K with a Radeon R9 380.

Piglit hangs randomly on this system both with and without the patch
so i could not make a comparison.

v2: fixed whitespace

Signed-off-by: Nils Wallménius <nils.wallmenius@gmail.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>