virgl: enable ARB_gpu_shader_fp64

This enables ARB_gpu_shader_fp64 if the host provides it.

Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>

docs/features.txt

   - Enhanced per-sample shading                         DONE ()
   - Interpolation functions                             DONE ()
   - New overload resolution rules                       DONE
-  GL_ARB_gpu_shader_fp64                                DONE (i965/gen7+, llvmpipe, softpipe)
+  GL_ARB_gpu_shader_fp64                                DONE (i965/gen7+, llvmpipe, softpipe, virgl)
   GL_ARB_sample_shading                                 DONE (i965/gen6+, nv50, virgl)
   GL_ARB_shader_subroutine                              DONE (freedreno, i965/gen6+, nv50, llvmpipe, softpipe, swr, virgl)
   GL_ARB_tessellation_shader                            DONE (i965/gen7+)

src/gallium/drivers/virgl/virgl_screen.c

       return vscreen->caps.caps.v1.bset.transform_feedback_overflow_query;
    case PIPE_CAP_SHADER_BUFFER_OFFSET_ALIGNMENT:
       return vscreen->caps.caps.v2.shader_buffer_offset_alignment;
+   case PIPE_CAP_DOUBLES:
+      return vscreen->caps.caps.v1.bset.has_fp64;
    case PIPE_CAP_TEXTURE_GATHER_SM5:
    case PIPE_CAP_BUFFER_MAP_PERSISTENT_COHERENT:
    case PIPE_CAP_FAKE_SW_MSAA:
    case PIPE_CAP_POLYGON_MODE_FILL_RECTANGLE:
    case PIPE_CAP_SPARSE_BUFFER_PAGE_SIZE:
    case PIPE_CAP_TGSI_BALLOT:
-   case PIPE_CAP_DOUBLES:
    case PIPE_CAP_TGSI_TES_LAYER_VIEWPORT:
    case PIPE_CAP_CAN_BIND_CONST_BUFFER_AS_VERTEX:
    case PIPE_CAP_ALLOW_MAPPED_BUFFERS_DURING_EXECUTION: