Restore fog in vulkan mobile renderer.

The condition accidentally disabled fog whenever it was enabled

doc/classes/Environment.xml

@@ -311,6 +311,7 @@
 		</member>
 		<member name="volumetric_fog_enabled" type="bool" setter="set_volumetric_fog_enabled" getter="is_volumetric_fog_enabled" default="false">
 			Enables the volumetric fog effect. Volumetric fog uses a screen-aligned froxel buffer to calculate accurate volumetric scattering in the short to medium range. Volumetric fog interacts with [FogVolume]s and lights to calculate localized and global fog. Volumetric fog uses a PBR single-scattering model based on extinction, scattering, and emission which it exposes to users as density, albedo, and emission.
+			[b]Note:[/b] Volumetric fog is only available in the forward plus renderer. It is not available in the mobile renderer or the compatibility renderer.
 		</member>
 		<member name="volumetric_fog_gi_inject" type="float" setter="set_volumetric_fog_gi_inject" getter="get_volumetric_fog_gi_inject" default="1.0">
 			Scales the strength of Global Illumination used in the volumetric fog. A value of [code]0.0[/code] means that Global Illumination will not impact the volumetric fog.

servers/rendering/renderer_rd/forward_mobile/render_forward_mobile.cpp

@@ -719,7 +719,7 @@ void RenderForwardMobile::_render_scene(RenderDataRD *p_render_data, const Color
 			spec_constant_base_flags |= 1 << SPEC_CONSTANT_DISABLE_DIRECTIONAL_LIGHTS;
 		}
 
-		if (!is_environment(p_render_data->environment) || environment_get_fog_enabled(p_render_data->environment)) {
+		if (!is_environment(p_render_data->environment) || !environment_get_fog_enabled(p_render_data->environment)) {
 			spec_constant_base_flags |= 1 << SPEC_CONSTANT_DISABLE_FOG;
 		}
 	}