nv50,nvc0: mark RGBX_UINT formats as renderable

This helps st/mesa avoid some (apparently) buggy fallbacks. Specifically
the CopyTexSubImage fallback tries to read texture A as RGBA_FLOAT and
write back that data into the target format, which fails for integer
formats which have no appropriate logic to do the conversion.

Since integer formats don't blend, there's no harm in the fact that the
"A" component gets written anyways.

Fixes, among others:
  https://www.khronos.org/registry/webgl/sdk/tests/conformance2/textures/canvas/tex-2d-rgb8ui-rgb_integer-unsigned_byte.html



Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit 78d3640e)