radeonsi: add support for PIPE_CAP_TGSI_TEXCOORD

radeonsi: add support for PIPE_CAP_TGSI_TEXCOORD

Without it, texcoords are mapped to GENERIC[0..7], PointCoord is mapped to
GENERIC[8], and user-defined varyings start from GENERIC[9]. Since texcoords
can only be used between VS and PS, and PointCoord is PS-only, it's silly to
always start from GENERIC[9] in all other shaders (such as LS, HS, ES, GS).

This adds support for TEXCOORD and PCOORD semantics. As a result, st/mesa
will use GENERIC[0] as a base for user-defined varyings, which should make
linking ES and GS as well as tessellation shaders at runtime easier.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>