drm/i915/dp: For PCON TMDS mode set only the relavant bits in config DPCD

Currently we reset the whole PCON linkConfig DPCD to set the TMDS mode.
This also resets the Source control bit and HDMI link enable bit and
goes to autonomous mode of operation, which is seen to spoil the PCONs
internal state.

This patch avoids resetting the PCON link config register and sets only
the source control bit, with FRL Enable bit set to 0 (TMDS mode) in the
configuration DPCD. It then enables the HDMI Link Enable bit.

v2: Removed the redundant resetting of the bits as the buffer is already
initialized to 0. (Uma)
Updated comments and commit message.

v3: Rebase

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110072947.171659-3-ankit.k.nautiyal@intel.com