drm/mgag200: Hardware cursor support

G200 cards support, at best, 16 colour palleted images for the cursor
so we do a conversion in the cursor_set function, and reject cursors
with more than 16 colours, or cursors with partial transparency. Xorg
falls back gracefully to software cursors in this case.

We can't disable/enable the cursor hardware without causing momentary
corruption around the cursor. Instead, once the cursor is on we leave
it on, and simulate turning the cursor off by moving it
offscreen. This works well.

Since we can't disable -> update -> enable the cursors, we double
buffer cursor icons, then just move the base address that points to
the old cursor, to the new. This also works well, but uses an extra
page of memory.

The cursor buffers are lazily-allocated on first cursor_set. This is
to make sure they don't take priority over any framebuffers in case of
limited memory.

Here is a representation of how the bitmap for the cursor is mapped in G200 memory :

  Each line of color cursor use 6 Slices of 8 bytes. Slices 0 to 3
  are used for the 4bpp bitmap, slice 4 for XOR mask and slice 5 for
  AND mask. Each line has the following format:

      //      Byte 0  Byte 1  Byte 2  Byte 3  Byte 4  Byte 5  Byte 6 Byte 7
      //
      // S0:  P00-01  P02-03  P04-05  P06-07  P08-09  P10-11  P12-13 P14-15
      // S1:  P16-17  P18-19  P20-21  P22-23  P24-25  P26-27  P28-29 P30-31
      // S2:  P32-33  P34-35  P36-37  P38-39  P40-41  P42-43  P44-45 P46-47
      // S3:  P48-49  P50-51  P52-53  P54-55  P56-57  P58-59  P60-61 P62-63
      // S4:  X63-56  X55-48  X47-40  X39-32  X31-24  X23-16  X15-08 X07-00
      // S5:  A63-56  A55-48  A47-40  A39-32  A31-24  A23-16  A15-08 A07-00
      //
      //       S0 to S5      = Slices 0 to 5
      //       P00 to P63    = Bitmap - pixels 0 to 63
      //       X00 to X63    = always 0 - pixels 0 to 63
      //       A00 to A63    = transparent markers - pixels 0 to 63
      //                       1 means colour, 0 means transparent

Signed-off-by: Christopher Harvey <charvey@matrox.com>
Signed-off-by: Mathieu Larouche <mathieu.larouche@matrox.com>
Acked-by: Julia Lemire <jlemire@matrox.com>
Tested-by: Julia Lemire <jlemire@matrox.com>
Signed-off-by: Dave Airlie <airlied@gmail.com>