r300: only enable half floats on r500s.

Thank to MAD for point it out.

src/mesa/drivers/dri/r300/r300_context.c

@@ -97,7 +97,6 @@ static const struct dri_extension card_extensions[] = {
   /* *INDENT-OFF* */
   {"GL_ARB_depth_texture",		NULL},
   {"GL_ARB_fragment_program",		NULL},
-  {"GL_ARB_half_float_vertex",		NULL},
   {"GL_ARB_occlusion_query",		GL_ARB_occlusion_query_functions},
   {"GL_ARB_multitexture",		NULL},
   {"GL_ARB_point_parameters",		GL_ARB_point_parameters_functions},
@@ -452,6 +451,8 @@ static void r300InitGLExtensions(GLcontext *ctx)
 	if (!r300->radeon.radeonScreen->drmSupportsOcclusionQueries) {
 		_mesa_disable_extension(ctx, "GL_ARB_occlusion_query");
 	}
+	if (r300->radeon.radeonScreen->chip_family >= CHIP_FAMILY_RV515)
+  		_mesa_enable_extension(ctx, "GL_ARB_half_float_vertex");
 }
 
 static void r300InitIoctlFuncs(struct dd_function_table *functions)