drm/panel: truly: Add additional delay after pulling down reset gpio

MTP SDM845 panel seems to need additional delay to bring panel
to a workable state. Running modetest without this change displays
blurry artifacts.

Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190527102616.28315-1-vivek.gautam@codeaurora.org