drm/i915/ddi: prefer poll_timeout_us() over readx_poll_timeout()

drm/i915/ddi: prefer poll_timeout_us() over readx_poll_timeout()

Unify on using poll_timeout_us() throughout instead of mixing with
readx_poll_timeout(). While the latter can be ever so slightly simpler,
they are both complicated enough that it's better to unify on one
approach only.

While at it, better separate the handling of error returns from
drm_dp_dpcd_readb() and the actual status byte. This is best achieved by
inlining the read_fec_detected_status() function, and switching to
drm_dp_dpcd_read_byte().

v2: Use drm_dp_dpcd_read_byte() (Imre)

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/63b10a36c7ab545c640b24bc8fc007ce2ea74623.1756383233.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>