setup-gummiboot-conf: Add HDMI/DP specific splash .conf entries

setup-gummiboot-conf: Add HDMI/DP specific splash .conf entries

Unused digital display outputs cause a kernel boot timeout that adds
to system boot up time:

"[drm] GMBUS [i915 gmbus dpb] timed out, falling back to bit banging on pin 5"

The problem goes away by disabling unused outputs: video:<output>:d.

However, the problem gets complicated since VTC1010 provides only DP-1 connector,
and MinnowBoard MAX provides only HDMI-A-1 connector. The solution is to generate
HDMI and DP specific splash .conf entries. The desired config entry can be enabled
based on the platform.

Change-Id: I8544ee7cf15d7b4fb8837147e5149cca2f8c6b7e
Bug-Tizen: TC-2195
Signed-off-by: Mikko Ylinen <mikko.ylinen@intel.com>