i965/vec4: use attribute slots to calculate URB read length
i965/vec4: use attribute slots to calculate URB read length

Do not use total attributes because a dvec3/dvec4 attribute requires two
slots. So rather use total attribute slots.

v2: do not use loop to calculate required attribute slots (Kenneth
Graunke)

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler