r300-gallium: organize fragment/vertex shaders

r300-gallium: organize fragment/vertex shaders

Appart from separating r3xx/r5xx fragment shaders, a more consistent
naming scheme has been applied. From now on:
r300 = all chips
r3xx = R300/R400 only
r5xx = R500 only
This way r300_fragment_shader is the master struct, and the structs
r3xx_fragment_shader and r5xx_fragment_shader inherits it.