gallium: use enums in p_defines.h (v2)
gallium: use enums in p_defines.h (v2)

and remove number assignments which are consecutive

Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com> (v1)
Reviewed-by: Roland Scheidegger <sroland@vmware.com> (v1)
Acked-by: Jose Fonseca <jfonseca@vmware.com> (v1)

v2: name enums

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler