etnaviv: Emit PE.ALPHA_COLOR_EXT* on GPUs with half-float support

etnaviv: Emit PE.ALPHA_COLOR_EXT* on GPUs with half-float support

At least GC880 (iMX6S), GC2000 (iMX6Q) blobs do not emit the
PE.ALPHA_COLOR_EXT0 and PE.ALPHA_COLOR_EXT1 into the command
stream. The GCnano (STM32MP1) is not affected by this change
either. This is because neither of these GPUs support the
half-float feature.

Emit PE.ALPHA_COLOR_EXT* in etnaviv only if half-float support
is present in the GPU. This fixes all of the currently failing
dEQPs in this group:
  dEQP-GLES2.functional.fragment_ops.blend.*

Fixes: 76adf041f25 ("etnaviv: fix blend color on newer GPUs")
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Jonathan Marek <jonathan@marek.ca>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4277>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4277>