drm/i2c: tda998x: fix EDID reading on TDA19988 devices

TDA19988 devices need their RAM enabled in order to read EDID
information.  Add support for this.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Rob Clark <robdclark@gmail.com>
Tested-by: Darren Etheridge <detheridge@ti.com>
Tested-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Tested-by: Russell King <rmk_kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>