virgl: modify how we handle GL_MAP_FLUSH_EXPLICIT_BIT

virgl: modify how we handle GL_MAP_FLUSH_EXPLICIT_BIT

Previously, we ignored the the glUnmap(..) operation and
flushed before we flush the cbuf.  Now, let's just flush
the data when we unmap.

Neither method is optimal, for example:

glMapBufferRange(.., 0, 100, GL_MAP_FLUSH_EXPLICIT_BIT)
glFlushMappedBufferRange(.., 25, 30)
glFlushMappedBufferRange(.., 65, 70)

We'll end up flushing 25 --> 70.  Maybe we can fix this later.

v2: Add fixme comment in the code (Elie)

Reviewed-by: Elie Tournier <elie.tournier@collabora.com>