gallium/draw: Fix enum type taken by draw_get_shader_param().

Pointed out by clang:

src/gallium/auxiliary/draw/draw_context.h:251:41: warning: implicit conversion
from enumeration type 'enum pipe_cap' to different enumeration type
'enum pipe_shader_cap' [-Wconversion]
      return tgsi_exec_get_shader_param(param);
             ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~~~~

src/gallium/auxiliary/draw/draw_context.h

@@ -243,7 +243,7 @@ boolean draw_need_pipeline(const struct draw_context *draw,
                            unsigned prim );
 
 static INLINE int
-draw_get_shader_param(unsigned shader, enum pipe_cap param)
+draw_get_shader_param(unsigned shader, enum pipe_shader_cap param)
 {
    switch(shader) {
    case PIPE_SHADER_VERTEX: