drm/i915: refactor transcoders reporting on error state

Instead of keeping track of the number of transcoders, loop through all
the interesting ones and check if there is a correspondent offset.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190222230254.20351-1-lucas.demarchi@intel.com