fix(input): placeholder text is hidden by parent visibility

[robertmesserle]

Previously, adding visibility: hidden to a parent element would not hide the placeholder text for md-input.

Closes #670

R: @hansl @jelbourn

[robertmesserle]

@hansl Doesn't look like there were any transitions used on visibility: hidden, seems identical with display: none

[hansl]

If you don't see any problems, then LGTM :)

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