patch 9.1.1147: preview-window does not scroll correctly

patch 9.1.1147: preview-window does not scroll correctly

Problem:  preview-window does not scroll correctly
Solution: init firstline = 0 for a preview window
          (Girish Palya)

The 'info' window, which appears during insert-mode completion to display
additional information, was not scrolling properly when using commands like:
	win_execute(popup_findinfo(), "normal! \<PageDown>")
This issue made it impossible to navigate through info window contents using
keyboard-based scrolling.
The fix correctly updates the w_firstline value of the popup window, ensuring
proper scrolling behavior. Mouse scrolling was already working as expected and
remains unaffected.

closes: #16703

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>