Fix initial placeholder in multiline TextInput

Summary:
Changelog: [internal]

`[self _invalidatePlaceholderVisibility]` wasn't triggered in multiline text input in Fabric. Even in Paper it was triggered what to me seems like coincidence rather than intention (I might be wrong). This is more explicit, visibility of placeholder text needs to be re-evaluated every time placeholder text is changed.

Reviewed By: shergin

Differential Revision: D26172754

fbshipit-source-id: 3a767d333b79c266a3d70a96883b1289fff16750

Libraries/Text/TextInput/Multiline/RCTUITextView.m

@@ -282,6 +282,7 @@ - (void)_invalidatePlaceholderVisibility
 - (void)_updatePlaceholder
 {
  _placeholderView.attributedText = [[NSAttributedString alloc] initWithString:_placeholder ?: @"" attributes:[self _placeholderTextAttributes]];
+ [self _invalidatePlaceholderVisibility];
 }
 
 - (NSDictionary<NSAttributedStringKey, id> *)_placeholderTextAttributes