pixman/pixman-vmx.c: get rid of warnings, and simplify somewhat

• Re-implement create_mask_32_128() based on suggestion from @siamashka. This lets the compiler pick the vector instruction to use which is usually the best idea.
• Use create_mask_32_128() instead of create_mask_1x32_128() in vmx_fill(), avoiding loading memory beyond the filler argument on the stack. Remove the now-unused create_mask_1x32_128(). This gets rid of some (correct) warnings from the compiler about indexing beyond the variable in question.
• Simplify the code in scaled_nearest_scanline_vmx_8888_8888_OVER(), Since combine4() does not take vector variables as arguments, there's no need to use a vector variable and casts back and forth to normal scalars for the arguments.