r600g: get rid of dummy pixel shader

r600g: get rid of dummy pixel shader

The idea is taken from radeonsi. The code mostly was already checking for null
pixel shader, so little checks had to be added.

Interestingly, acc. to testing with GTAⅣ, though binding of null shader happens
a lot at the start (then just stops), but draw_vbo() never actually sees null
ps.

v2: added a check I missed because of a macros using a prefix to choose
a shader.

Signed-off-by: Constantine Kharlamov <Hi-Angel@yandex.ru>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>