Claude review: drm/dp: Add max bpp delta computation constants

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Issues found:**

1. **YCbCr422 mask placed under wrong register.** The commit message and version notes say the 422 mask was added "under 0x6E register," but the code places it under `DP_DSC_BITS_PER_PIXEL_INC` (0x06F):

   ```c
   #define DP_DSC_BITS_PER_PIXEL_INC           0x06F
   ...
   # define DP_DSC_NATIVE_YCbCr422_MAX_BPP_DELTA_MASK  0x78
   # define DP_DSC_BPP_DELTA_NATIVE_422		16
   ```

   If this mask really belongs to register 0x06E (as the commit message implies), it should be grouped with the other masks under `DP_DSC_MAX_BPP_DELTA_VERSION_1`. If it genuinely belongs to 0x06F (bits [6:3] of the BPP increment register), then the commit message is wrong. Either way, something needs to be reconciled with the DP spec.

2. **Inconsistent naming convention.** The 444/420 constants are named `DP_DSC_BPP_DELTA_444` and `DP_DSC_BPP_DELTA_420`, but the 422 constant is `DP_DSC_BPP_DELTA_NATIVE_422`. The naming should be consistent -- either all include `NATIVE` or none do.

3. **Unused definitions.** `DP_DSC_NATIVE_YCbCr422_MAX_BPP_DELTA_MASK` and `DP_DSC_BPP_DELTA_NATIVE_422` are defined but not used anywhere in the series. If YCbCr422 support isn't being added yet, it may be better to defer these definitions to when they're actually needed, or at minimum note in the commit message that they're being added for future use.

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