drm: Make drm.debug parameter description more helpful

Let's be user-friendly and print an actually helpful parameter
description.

This makes modinfo output the debug parameter like this:

parm:           debug:Enable debug output, where each bit enables a debug category.
		Bit 0 (0x01) will enable CORE messages (drm core code)
		Bit 1 (0x02) will enable DRIVER messages (drm controller code)
		Bit 2 (0x04) will enable KMS messages (modesetting code)
		Bit 3 (0x08) will enable PRIME messages (prime code)
		Bit 4 (0x10) will enable ATOMIC messages (atomic code)
		Bit 5 (0x20) will enable VBL messages (vblank code) (int)

Changes from v1:

  * Fixed s/PRMIE/PRIME typo.
  * Add ATOMIC and VBL debug parameter documentation.
  * Prefix the continuation lines with two tabs and
    removed the last new line.
  * Remove spurious whitespace.

Signed-off-by: Ezequiel Garcia <[email protected]>
Reviewed-by: Jani Nikula <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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