Radeon Rv790 "Forbidden register 0x0030 in cs at 9"

[Andy Furniss]

Noticed this in dmesg today, it doesn't seem to break anything and it's 
not reproducable.

[ 2605.261326] Forbidden register 0x0030 in cs at 9
[ 2605.261335] [drm:radeon_cs_ib_chunk] *ERROR* Invalid command stream !

Seems it was triggered by mplayer using vdpau just for display, though 
grepping kern.log only found one other on Oct 22nd.

mplayer has been used far more than just twice in this period.

Running drm-fixes today and drm-next on the 22nd.