drm/radeon: Fix eDP for single-display iMac11,2

drm/radeon: Fix eDP for single-display iMac11,2

Apple iMac11,2 (mid 2010) also with Radeon HD-4670 that has the same
issue as iMac10,1 (late 2009) where the internal eDP panel stays dark on
driver load.  This patch treats iMac11,2 the same as iMac10,1,
so the eDP panel stays active.

Additional steps:
Kernel boot parameter radeon.nomodeset=0 required to keep the eDP
panel active.

This patch is an extension of
commit 564d8a2cf3ab ("drm/radeon: Fix eDP for single-display iMac10,1 (v2)")
Link: https://lore.kernel.org/all/lsq.1507553064.833262317@decadent.org.uk/
Signed-off-by: Mark Hawrylak <mark.hawrylak@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org