This decreases the release libgallium_dri.so size without debug symbols
by 16384 bytes. The CPU time spent in si_emit_draw_packets decreased
from 4.5% to 4.1% in viewperf13/catia/plane01.

The previous code did:
    cs->current.buf[cs->current.cdw++] = ...;
    cs->current.buf[cs->current.cdw++] = ...;
    cs->current.buf[cs->current.cdw++] = ...;
    cs->current.buf[cs->current.cdw++] = ...;

The new code does:
    unsigned num = cs->current.cdw;
    uint32_t *buf = cs->current.buf;
    buf[num++] = ...;
    buf[num++] = ...;
    buf[num++] = ...;
    buf[num++] = ...;
    cs->current.cdw = num;

The code is the same (radeon_emit is redefined as a macro) except that
all set and emit functions must be surrounded by radeon_begin(cs) and
radeon_end().

radeon_packets_added() returns whether there has been any new packets added
since radeon_begin.

radeon_end_update_context_roll(sctx) sets sctx->context_roll = true
if there has been any new packets added since radeon_begin.

For now, the "cs" parameter is intentionally unused in radeon_emit and
radeon_emit_array.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <!8653>