drm/i915: Only enable cursor if it can be enabled.

drm/i915: Only enable cursor if it can be enabled.

The cursor should only be enabled if it's visible. This fixes
igt/kms_cursor_crc, which may otherwise produce the following
warning:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 3425 at drivers/gpu/drm/i915/intel_display.c:9995 intel_crtc_update_cursor+0x14c/0x4d0 [i915]()
Missing switch case (0) in i9xx_update_cursor
Modules linked in: i915
CPU: 0 PID: 3425 Comm: kms_cursor_crc Tainted: G        W       4.1.0-rc7-patser+ #4079
Hardware name: LENOVO 2349AV8/2349AV8, BIOS G1ETA5WW (2.65 ) 04/15/2014
 ffffffffc01aad10 ffff8800b083faa8 ffffffff817f7827 0000000080000001
 ffff8800b083faf8 ffff8800b083fae8 ffffffff81084955 ffff8800b083fad8
 ffff8800c4931148 0000000001200000 ffff8800c48b0000 0000000000000000
Call Trace:
 [<ffffffff817f7827>] dump_stack+0x4f/0x7b
 [<ffffffff81084955>] warn_slowpath_common+0x85/0xc0
 [<ffffffff810849d1>] warn_slowpath_fmt+0x41/0x50
 [<ffffffffc0139f2c>] intel_crtc_update_cursor+0x14c/0x4d0 [i915]
 [<ffffffffc01497f4>] __intel_set_mode+0x6c4/0x750 [i915]
 [<ffffffffc0150143>] intel_crtc_set_config+0x473/0x5c0 [i915]
 [<ffffffff81467da9>] drm_mode_set_config_internal+0x69/0x120
 [<ffffffff8146c1b9>] drm_mode_setcrtc+0x189/0x540
 [<ffffffff8145c7e0>] drm_ioctl+0x1a0/0x6a0
 [<ffffffff810b3b41>] ? get_parent_ip+0x11/0x50
 [<ffffffff811e9c28>] do_vfs_ioctl+0x2f8/0x530
 [<ffffffff810d0f7d>] ? trace_hardirqs_on+0xd/0x10
 [<ffffffff812e7746>] ? selinux_file_ioctl+0x56/0x100
 [<ffffffff811e9ee1>] SyS_ioctl+0x81/0xa0
 [<ffffffff81801617>] system_call_fastpath+0x12/0x6f
---[ end trace abf0f71163290a96 ]---

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>