drm/i915/bios: Fix the sequence size calculations for MIPI seq v3

Two errors in a single line. The size was read from the wrong offset,
and the end index didn't take the five bytes for sequence byte and size
of sequence into account. Fix it all, and break up the calculations a
bit to make it clearer.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reported-and-tested-by: Mika Kahola <mika.kahola@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Fixes: 2a33d934

 ("drm/i915/bios: add support for MIPI sequence block v3")
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452784327-27258-1-git-send-email-jani.nikula@intel.com