nir: lima: Add gl_PointCoord handling

This introduces gl_PointCoord handling in lima. To be able to easily distingish between gl_PointCoord and a regular varying in ppir, we handle gl_PointCoord as a system value. https://github.com/cwabbott0/mali-isa-docs/blob/master/Utgard-PP.md mentions, that the following transformation has to be done by the shader:

uniform vec4 gl_mali_PointCoordScaleBias; //created by the compiler
gl_PointCoord = gl_mali_PointCoordScaleBias.zw + gl_mali_PointCoordScaleBias.xy * gl_PointCoord_orig;

However, i didn't stumble across any disassemble code which is produced by the mali offline compiler and let's me see gl_mali_PointCoordScaleBias. I discussed that on irc with @cwabbott0 and iirc it has sth. to do with prerotation functionality. We agreed to ignore that atm and just use the intrinsic variable as is. lima NIR