drm/i915: enable error detection & state collection

This patch enables error detection by enabling several types of error
interrupts.  When an error interrupt is received, the interrupt
handler captures the error state; hopefully resulting in an accurate
set of error data (error type, active head pointer, etc.).  The new
record is then available from sysfs.  The current code will also dump
the error state to the system log.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>