radeonsi: Release storage for smda_uploads when the context is destroyed

This fixes a memory leak in the flush code:

Direct leak of 128 byte(s) in 1 object(s) allocated from: #0 in __interceptor_realloc .../gcc-8.3.0/libsanitizer/asan/asan_malloc_linux.cc:105 #1 (closed) in si_buffer_do_flush_region src/gallium/drivers/radeonsi/si_buffer.c:573 #2 (closed) in si_buffer_flush_region src/gallium/drivers/radeonsi/si_buffer.c:608 #3 (closed) in si_buffer_flush_region src/gallium/drivers/radeonsi/si_buffer.c:597