radeonsi: Release storage for smda_uploads when the context is destroyed

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 in si_buffer_do_flush_region src/gallium/drivers/radeonsi/si_buffer.c:573
    #2 in si_buffer_flush_region src/gallium/drivers/radeonsi/si_buffer.c:608
    #3 in si_buffer_flush_region src/gallium/drivers/radeonsi/si_buffer.c:597

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>