drm/i915: Simplify i915_reg_read_ioctl

Convert to use the freshly available made INTEL_GEN_MASK for easier
grepping and improve function readability and clarify the UABI
documentation.

No functional changes.

v2:
- Lift GEM_BUG_ONs and use is_power_of_2 (Chris)
- Retain -EINVAL on bad flags behavior (Chris)

v3:
- Extract flags with 'entry->size - 1' (Chris)

v4:
- Add GEM_BUG_ON on for flags vs entry offset (Chris)

v5:
- Use 'u16' to match 'dev_priv' (Ville)

v6:
- Fix checkpatch.pl errors

Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170913115255.13851-2-joonas.lahtinen@linux.intel.com

include/uapi/drm/i915_drm.h

@@ -1308,14 +1308,16 @@ struct drm_i915_reg_read {
 	 * be specified
 	 */
 	__u64 offset;
+#define I915_REG_READ_8B_WA BIT(0)
+
 	__u64 val; /* Return value */
 };
 /* Known registers:
  *
  * Render engine timestamp - 0x2358 + 64bit - gen7+
  * - Note this register returns an invalid value if using the default
- *   single instruction 8byte read, in order to workaround that use
- *   offset (0x2538 | 1) instead.
+ *   single instruction 8byte read, in order to workaround that pass
+ *   flag I915_REG_READ_8B_WA in offset field.
  *
  */