Claude review: drm/ast: Use constants for MCR registers

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

Patch Review

This is the largest mechanical patch (~300 line changes in ast_2300.c alone). The conversion was done via grep+sed plus manual fixes, which is appropriate for this kind of bulk replacement. The register constant definitions in `ast_reg.h` are well-organized.

Minor nit: In `set_mpll_2500()` there's a loop that still uses raw addresses:
```c
for (addr = 0x1e6e0004; addr < 0x1e6e0090;) {
    ast_moutdwm(ast, addr, 0x0);
    addr += 4;
}
```
This loop wasn't converted to use constants (it iterates through a range), which is fine - converting a loop like this would be awkward.

One concern: in `ast_dp501.c`, the P2A bridge setup at `ast_init_analog()` and `ast_init_dvo()` writes `AST_REG_MCR00` to register `0xf004`. This is the P2A base register being set to the MCR base, but using the MCR constant name for the *value* written is semantically correct (the MCR base address happens to equal `AST_REG_MCR00`).

However, in `ast_drv.c:ast_detect_chip()`, the same pattern `__ast_write32(regs, 0xf004, AST_REG_MCR00)` is used - where the intent is to set the P2A window base to the MCR region. Using `AST_REG_MCR00` as the *value* is technically correct but conceptually it's the base address being set. The later patches fix this by using `__ast_mindwm` which handles the segment setup internally.

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