etnaviv: enable triangle strips only when the hardware supports it

etnaviv: enable triangle strips only when the hardware supports it

Some hardware has a bug with triangle strips and it is signalled by the
flag BUG_FIXED8 whether this bug has been fixed. So only enable triangle
strips when this flag is set.

Thanks: Jonathan Marek and Christian Gmeiner for the pointers

v2: Add TODO to indicate that the handling should be refined
    (Jonathan & Christian)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>