i965: fix fetching constants from constant buffer in glsl path

i965: fix fetching constants from constant buffer in glsl path

the driver used to overwrite grf0 then use implicit move by send instruction
to move contents of grf0 to mrf1. However, we must not overwrite grf0 since
it's still used later for fb write.
Instead, do the move directly do mrf1 (we could use implicit move from another
grf reg to mrf1 but since we need a mov to encode the data anyway it doesn't
seem to make sense).
I think the dp_READ/WRITE_16 functions may suffer from the same issue.
While here also remove unnecessary msg_reg_nr parameter from the dataport
functions since always message register 1 is used.