i965: Don't use PREAD for glGetBufferSubData().

Just map the buffer and memcpy.  This will do a CPU mmap, which should
be reasonably efficient, and doing this gives us full control over the
domains and caching instead of leaving it to the kernel.

This prevents regressions on Braswell in the next commit.  Specifically
GL45-CTS.shader_atomic_counters.basic-buffer-operations.  Because async
maps start skipping set-domain, the pread thought everything was nicely
still in the CPU domain, and returned stale data.

v2: Use _mesa_error_no_memory() if the map fails instead of crashing.

Reviewed-by: Chris Wilson <[email protected]>

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