panfrost: Advertise FRAMEBUFFER_NO_ATTACHMENTS

Passes dEQP-GLES31.functional.fbo.no_attachments.*, it doesn't look like
we need to do anything special.
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <!9105>

docs/features.txt

@@ -248,7 +248,7 @@ GLES3.1, GLSL ES 3.1 -- all DONE: i965/hsw+, nvc0, r600, radeonsi, virgl, v3d, s
   GL_ARB_compute_shader                                 DONE (freedreno/a5xx+, i965/gen7+, panfrost)
   GL_ARB_draw_indirect                                  DONE (freedreno, i965/gen7+, swr)
   GL_ARB_explicit_uniform_location                      DONE (all drivers that support GLSL)
-  GL_ARB_framebuffer_no_attachments                     DONE (freedreno, i965/gen7+, softpipe)
+  GL_ARB_framebuffer_no_attachments                     DONE (freedreno, i965/gen7+, softpipe, panfrost)
   GL_ARB_program_interface_query                        DONE (all drivers)
   GL_ARB_shader_atomic_counters                         DONE (freedreno/a5xx+, i965/gen7+, swr, panfrost/midgard)
   GL_ARB_shader_image_load_store                        DONE (freedreno/a5xx+, i965/gen7+, swr, panfrost/midgard)

src/gallium/drivers/panfrost/pan_screen.c

@@ -116,6 +116,7 @@ panfrost_get_param(struct pipe_screen *screen, enum pipe_cap param)
         case PIPE_CAP_FRONTEND_NOOP:
         case PIPE_CAP_SAMPLE_SHADING:
         case PIPE_CAP_FRAGMENT_SHADER_DERIVATIVES:
+        case PIPE_CAP_FRAMEBUFFER_NO_ATTACHMENT:
                 return 1;
 
         case PIPE_CAP_MAX_RENDER_TARGETS: