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

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);
             ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^~~~~