etnaviv: tgsi: Fix gl_FrontFacing support

etnaviv: tgsi: Fix gl_FrontFacing support

The GPU presents the state of the hardware front_face in internal
register 0 (i0), the range of which is 0.0f..1.0f.

This patch assigns the fragment shader input to this internal register.
Moreover, based on the internal front_ccw state, the value of the i0
register is inverted accordingly using SET.EQ/SEQ.NE instruction before
being further processed in the shader. This mimics the operation of the
NIR compiler.

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/2868>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2868>