freedreno/turnip: Don't request fragcoord components we don't use
Noticed while looking at another issue:
Right now we ask for the full vec4 of frag coord when it has been used in the shader, but the HW has independent bits for each component in RB_RENDER_CONTROL0/GRAS_CNTL. Possibly performance-relevant?
Relevant test case is dEQP-GLES2.functional.shaders.builtin_variable.fragcoord_xyz and blob only requests XYZ.