[Intel-gfx] [Patch] Add minimum watermark level for I830

[Thomas Richter]

Dear Daniel, dear intel experts,

please find a minimally-invasive patch to add a minimum watermark level 
to the current watermark logic. This fixes the flickering and video 
overlay crashes on 830M(G) chipsets. Note that this patch does not alter 
the watermark algorithm on any other family of the intel chipsets.

Greetings,
	Thomas

PS: What I do not understand about the current logic is that there is a 
"default watermark" level if the computed watermark is below zero, 
though computed watermarks can well drop below this default. The 
appended patch still allows that to be backwards compatible, though this 
construction sounds pretty weird to me.


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