fix(form): TextArea disabled styles

The placeholder text was still appearing for TextArea due to missing
some of the base text field styles. Since the code for the floating
state is the same between TextField and TextArea, the `&--floating`
state was moved to the `@mixin rmd-text-field-base`

packages/form/src/text-field/_mixins.scss

@@ -246,21 +246,6 @@
       @include rmd-theme(color, text-disabled-on-background);
     }
   }
-}
-
-/// @access private
-@mixin rmd-text-field {
-  @include rmd-utils-rtl {
-    @include rmd-form-theme(padding-left, text-padding-right);
-    @include rmd-form-theme(padding-right, text-padding-left);
-  }
-  @include rmd-form-theme(padding-left, text-padding-left);
-  @include rmd-form-theme(padding-right, text-padding-right);
-  @include rmd-form-theme(padding-top, text-padding-top);
-  @include rmd-text-field-base;
-
-  flex: 1 1 auto;
-  height: 100%;
 
   &--floating {
     @include rmd-text-field-placeholder {
@@ -282,6 +267,21 @@
   }
 }
 
+/// @access private
+@mixin rmd-text-field {
+  @include rmd-utils-rtl {
+    @include rmd-form-theme(padding-left, text-padding-right);
+    @include rmd-form-theme(padding-right, text-padding-left);
+  }
+  @include rmd-form-theme(padding-left, text-padding-left);
+  @include rmd-form-theme(padding-right, text-padding-right);
+  @include rmd-form-theme(padding-top, text-padding-top);
+  @include rmd-text-field-base;
+
+  flex: 1 1 auto;
+  height: 100%;
+}
+
 /// @access private
 @mixin rmd-text-field-addon {
   @include rmd-icon-theme(height, size);
@@ -354,19 +354,6 @@
   flex: 1 1 auto;
   height: 100%;
 
-  &--floating {
-    @include rmd-text-field-placeholder {
-      color: transparent;
-      transition: color $rmd-transition-standard-time;
-    }
-
-    &:focus {
-      @include rmd-text-field-placeholder {
-        @include rmd-theme(color, text-secondary-on-background);
-      }
-    }
-  }
-
   &--rh {
     resize: horizontal;
   }