panfrost: Compute viewport state on the fly

panfrost: Compute viewport state on the fly

Previously, we were caching this incorrectly; there's no real reason to
given how variable it is (sensitive to changes in viewport, framebuffer
dimensions, and scissors) and how cheap it is to recompute. So, just do
it on the fly each draw.

Fixes glmark-es2 -bshadow and -brefract.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>