drm/i915: avoid reading DP_TP_CTL twice

Just avoid the additional read in case DP_TP_CTL is enabled:
read it once and save the value.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191030012448.14937-5-lucas.demarchi@intel.com