drm: Use names of ioctls in debug traces

The intention here is to make the output of dmesg with full verbosity a
bit easier for a human to parse. This commit transforms:

[drm:drm_ioctl], pid=699, cmd=0x6458, nr=0x58, dev 0xe200, auth=1
[drm:drm_ioctl], pid=699, cmd=0xc010645b, nr=0x5b, dev 0xe200, auth=1
[drm:drm_ioctl], pid=699, cmd=0xc0106461, nr=0x61, dev 0xe200, auth=1
[drm:drm_ioctl], pid=699, cmd=0xc01c64ae, nr=0xae, dev 0xe200, auth=1
[drm:drm_mode_addfb], [FB:32]
[drm:drm_ioctl], pid=699, cmd=0xc0106464, nr=0x64, dev 0xe200, auth=1
[drm:drm_vm_open_locked], 0x7fd9302fe000,0x00a00000
[drm:drm_ioctl], pid=699, cmd=0x400c645f, nr=0x5f, dev 0xe200, auth=1
[drm:drm_ioctl], pid=699, cmd=0xc00464af, nr=0xaf, dev 0xe200, auth=1
[drm:intel_crtc_set_config], [CRTC:3] [NOFB]

into:

[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, I915_GEM_THROTTLE
[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, I915_GEM_CREATE
[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, I915_GEM_SET_TILING
[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, IOCTL_MODE_ADDFB
[drm:drm_mode_addfb], [FB:32]
[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, I915_GEM_MMAP_GTT
[drm:drm_vm_open_locked], 0x7fd9302fe000,0x00a00000
[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, I915_GEM_SET_DOMAIN
[drm:drm_ioctl], pid=699, dev=0xe200, auth=1, DRM_IOCTL_MODE_RMFB
[drm:intel_crtc_set_config], [CRTC:3] [NOFB]

v2: drm_ioctls is now a constant (Ville Syrjälä)

Signed-off-by: Chris Cummins <christopher.e.cummins@intel.com>