drm/amdgpu: Fix hdmi deep color support.

When porting the hdmi deep color detection code from
radeon-kms to amdgpu-kms apparently some kind of
copy and paste error happened, attaching an else
branch to the wrong if statement.

The result is that hdmi deep color mode is always
disabled, regardless of gpu and display capabilities and
user wishes, as the code mistakenly thinks that the display
doesn't provide the required max_tmds_clock limit and falls
back to 8 bpc.

This patch fixes deep color support, as tested on a
R9 380 Tonga Pro + suitable display, and should be
backported to all kernels with amdgpu-kms support.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: stable@vger.kernel.org
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>