radv: remove useless sync after CmdClear{Color,DepthStencil}Image()

radv: remove useless sync after CmdClear{Color,DepthStencil}Image()

'post_flush' is only set to NULL for the normal clear path
(ie. only vkCmdClearColorImage() and vkCmdClearDepthStencilImage()
are affected commands).

Because these two operations have to be externally synchronized
with VK_PIPELINE_STAGE_TRANSFER_BIT and VK_ACCESS_TRANSFER_WRITE_BIT,
it's useless to set those flags internallY.

VK_PIPELINE_STAGE_TRANSFER_BIT will wait for compute to be idle,
while VK_ACCESS_TRANSFER_WRITE_BIT will invalidate both L1 vector
caches and L2. RADV_CMD_FLAG_WRITEBACK_GLOBAL_L2 will be superseded
by RADV_CMD_FLAG_INV_GLOBAL_L2.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>