mx6sabresd: Avoid hang when HDMI cable is not connected

Since commit d9b89460 (mx6sabresd: Add LVDS splash screen support) the
following hang happens if the HDMI cable is not connected or the 'panel'
variable is not set:

U-Boot 2013.10-rc2-12978-g47ac53d-dirty (Sep 11 2013 - 15:07:38)

CPU:   Freescale i.MX6Q rev1.2 at 792 MHz
Reset cause: POR
Board: MX6-SabreSD
DRAM:  1 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1, FSL_SDHC: 2
...

Provide a check to 'dev->detect' in order to prevent the hang.

Reported-by: Pardeep Kumar Singla <b45784@freescale.com>
Suggested-by: Eric Bénard <eric@eukrea.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>