panfrost/midgard: Fix swizzle for store instructions

panfrost/midgard: Fix swizzle for store instructions

The current logic considers that the nir_intrinsic_component(store_intr)
encodes the source components start, but it actually encodes the
destination one. Source component offset adjustment is taken care of in
install_registers_instr(), when offset_swizzle() is called.

This fixes dEQP-GLES2.functional.shaders.random.all_features.fragment.45
when PAN_MESA_DEBUG=deqp (looks like exposing GLES3 features has an
impact on the varyings layout).

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3429>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3429>